diff --git a/Userland/Libraries/LibJS/Tests/builtins/Temporal/Instant/Instant.prototype.since.js b/Userland/Libraries/LibJS/Tests/builtins/Temporal/Instant/Instant.prototype.since.js
index fa806cbb01..6c1205712c 100644
--- a/Userland/Libraries/LibJS/Tests/builtins/Temporal/Instant/Instant.prototype.since.js
+++ b/Userland/Libraries/LibJS/Tests/builtins/Temporal/Instant/Instant.prototype.since.js
@@ -18,3 +18,173 @@ describe("errors", () => {
         }).toThrowWithMessage(TypeError, "Not an object of type Temporal.Instant");
     });
 });
+
+describe("rounding modes", () => {
+    const earlier = new Temporal.Instant(
+        217178610_123_456_789n /* 1976-11-18T15:23:30.123456789Z */
+    );
+    const later = new Temporal.Instant(
+        1572345998_271_986_289n /* 2019-10-29T10:46:38.271986289Z */
+    );
+    const largestUnit = "hours";
+
+    test("'ceil' rounding mode", () => {
+        const expected = [
+            ["hours", "PT376436H", "-PT376435H"],
+            ["minutes", "PT376435H24M", "-PT376435H23M"],
+            ["seconds", "PT376435H23M9S", "-PT376435H23M8S"],
+            ["milliseconds", "PT376435H23M8.149S", "-PT376435H23M8.148S"],
+            ["microseconds", "PT376435H23M8.14853S", "-PT376435H23M8.148529S"],
+            ["nanoseconds", "PT376435H23M8.1485295S", "-PT376435H23M8.1485295S"],
+        ];
+
+        const roundingMode = "ceil";
+        expected.forEach(([smallestUnit, expectedPositive, expectedNegative]) => {
+            const sincePositive = later.since(earlier, { largestUnit, smallestUnit, roundingMode });
+            expect(sincePositive.toString()).toBe(expectedPositive);
+
+            const sinceNegative = earlier.since(later, { largestUnit, smallestUnit, roundingMode });
+            expect(sinceNegative.toString()).toBe(expectedNegative);
+        });
+    });
+
+    test("'expand' rounding mode", () => {
+        const expected = [
+            ["hours", "PT376436H", "-PT376436H"],
+            ["minutes", "PT376435H24M", "-PT376435H24M"],
+            ["seconds", "PT376435H23M9S", "-PT376435H23M9S"],
+            ["milliseconds", "PT376435H23M8.149S", "-PT376435H23M8.149S"],
+            ["microseconds", "PT376435H23M8.14853S", "-PT376435H23M8.14853S"],
+            ["nanoseconds", "PT376435H23M8.1485295S", "-PT376435H23M8.1485295S"],
+        ];
+
+        const roundingMode = "expand";
+        expected.forEach(([smallestUnit, expectedPositive, expectedNegative]) => {
+            const sincePositive = later.since(earlier, { largestUnit, smallestUnit, roundingMode });
+            expect(sincePositive.toString()).toBe(expectedPositive);
+
+            const sinceNegative = earlier.since(later, { largestUnit, smallestUnit, roundingMode });
+            expect(sinceNegative.toString()).toBe(expectedNegative);
+        });
+    });
+
+    test("'floor' rounding mode", () => {
+        const expected = [
+            ["hours", "PT376435H", "-PT376436H"],
+            ["minutes", "PT376435H23M", "-PT376435H24M"],
+            ["seconds", "PT376435H23M8S", "-PT376435H23M9S"],
+            ["milliseconds", "PT376435H23M8.148S", "-PT376435H23M8.149S"],
+            ["microseconds", "PT376435H23M8.148529S", "-PT376435H23M8.14853S"],
+            ["nanoseconds", "PT376435H23M8.1485295S", "-PT376435H23M8.1485295S"],
+        ];
+
+        const roundingMode = "floor";
+        expected.forEach(([smallestUnit, expectedPositive, expectedNegative]) => {
+            const sincePositive = later.since(earlier, { largestUnit, smallestUnit, roundingMode });
+            expect(sincePositive.toString()).toBe(expectedPositive);
+
+            const sinceNegative = earlier.since(later, { largestUnit, smallestUnit, roundingMode });
+            expect(sinceNegative.toString()).toBe(expectedNegative);
+        });
+    });
+
+    test("'halfCeil' rounding mode", () => {
+        const expected = [
+            ["hours", "PT376435H", "-PT376435H"],
+            ["minutes", "PT376435H23M", "-PT376435H23M"],
+            ["seconds", "PT376435H23M8S", "-PT376435H23M8S"],
+            ["milliseconds", "PT376435H23M8.149S", "-PT376435H23M8.149S"],
+            ["microseconds", "PT376435H23M8.14853S", "-PT376435H23M8.148529S"],
+            ["nanoseconds", "PT376435H23M8.1485295S", "-PT376435H23M8.1485295S"],
+        ];
+
+        const roundingMode = "halfCeil";
+        expected.forEach(([smallestUnit, expectedPositive, expectedNegative]) => {
+            const sincePositive = later.since(earlier, { largestUnit, smallestUnit, roundingMode });
+            expect(sincePositive.toString()).toBe(expectedPositive);
+
+            const sinceNegative = earlier.since(later, { largestUnit, smallestUnit, roundingMode });
+            expect(sinceNegative.toString()).toBe(expectedNegative);
+        });
+    });
+
+    test("'halfEven' rounding mode", () => {
+        const expected = [
+            ["hours", "PT376435H", "-PT376435H"],
+            ["minutes", "PT376435H23M", "-PT376435H23M"],
+            ["seconds", "PT376435H23M8S", "-PT376435H23M8S"],
+            ["milliseconds", "PT376435H23M8.149S", "-PT376435H23M8.149S"],
+            ["microseconds", "PT376435H23M8.14853S", "-PT376435H23M8.14853S"],
+            ["nanoseconds", "PT376435H23M8.1485295S", "-PT376435H23M8.1485295S"],
+        ];
+
+        const roundingMode = "halfEven";
+        expected.forEach(([smallestUnit, expectedPositive, expectedNegative]) => {
+            const sincePositive = later.since(earlier, { largestUnit, smallestUnit, roundingMode });
+            expect(sincePositive.toString()).toBe(expectedPositive);
+
+            const sinceNegative = earlier.since(later, { largestUnit, smallestUnit, roundingMode });
+            expect(sinceNegative.toString()).toBe(expectedNegative);
+        });
+    });
+
+    test("'halfExpand' rounding mode", () => {
+        const expected = [
+            ["hours", "PT376435H", "-PT376435H"],
+            ["minutes", "PT376435H23M", "-PT376435H23M"],
+            ["seconds", "PT376435H23M8S", "-PT376435H23M8S"],
+            ["milliseconds", "PT376435H23M8.149S", "-PT376435H23M8.149S"],
+            ["microseconds", "PT376435H23M8.14853S", "-PT376435H23M8.14853S"],
+            ["nanoseconds", "PT376435H23M8.1485295S", "-PT376435H23M8.1485295S"],
+        ];
+
+        const roundingMode = "halfExpand";
+        expected.forEach(([smallestUnit, expectedPositive, expectedNegative]) => {
+            const sincePositive = later.since(earlier, { largestUnit, smallestUnit, roundingMode });
+            expect(sincePositive.toString()).toBe(expectedPositive);
+
+            const sinceNegative = earlier.since(later, { largestUnit, smallestUnit, roundingMode });
+            expect(sinceNegative.toString()).toBe(expectedNegative);
+        });
+    });
+
+    test("'halfFloor' rounding mode", () => {
+        const expected = [
+            ["hours", "PT376435H", "-PT376435H"],
+            ["minutes", "PT376435H23M", "-PT376435H23M"],
+            ["seconds", "PT376435H23M8S", "-PT376435H23M8S"],
+            ["milliseconds", "PT376435H23M8.149S", "-PT376435H23M8.149S"],
+            ["microseconds", "PT376435H23M8.148529S", "-PT376435H23M8.14853S"],
+            ["nanoseconds", "PT376435H23M8.1485295S", "-PT376435H23M8.1485295S"],
+        ];
+
+        const roundingMode = "halfFloor";
+        expected.forEach(([smallestUnit, expectedPositive, expectedNegative]) => {
+            const sincePositive = later.since(earlier, { largestUnit, smallestUnit, roundingMode });
+            expect(sincePositive.toString()).toBe(expectedPositive);
+
+            const sinceNegative = earlier.since(later, { largestUnit, smallestUnit, roundingMode });
+            expect(sinceNegative.toString()).toBe(expectedNegative);
+        });
+    });
+
+    test("'halfTrunc' rounding mode", () => {
+        const expected = [
+            ["hours", "PT376435H", "-PT376435H"],
+            ["minutes", "PT376435H23M", "-PT376435H23M"],
+            ["seconds", "PT376435H23M8S", "-PT376435H23M8S"],
+            ["milliseconds", "PT376435H23M8.149S", "-PT376435H23M8.149S"],
+            ["microseconds", "PT376435H23M8.148529S", "-PT376435H23M8.148529S"],
+            ["nanoseconds", "PT376435H23M8.1485295S", "-PT376435H23M8.1485295S"],
+        ];
+
+        const roundingMode = "halfTrunc";
+        expected.forEach(([smallestUnit, expectedPositive, expectedNegative]) => {
+            const sincePositive = later.since(earlier, { largestUnit, smallestUnit, roundingMode });
+            expect(sincePositive.toString()).toBe(expectedPositive);
+
+            const sinceNegative = earlier.since(later, { largestUnit, smallestUnit, roundingMode });
+            expect(sinceNegative.toString()).toBe(expectedNegative);
+        });
+    });
+});
diff --git a/Userland/Libraries/LibJS/Tests/builtins/Temporal/Instant/Instant.prototype.until.js b/Userland/Libraries/LibJS/Tests/builtins/Temporal/Instant/Instant.prototype.until.js
index 64ea328ca9..6c7b824fcd 100644
--- a/Userland/Libraries/LibJS/Tests/builtins/Temporal/Instant/Instant.prototype.until.js
+++ b/Userland/Libraries/LibJS/Tests/builtins/Temporal/Instant/Instant.prototype.until.js
@@ -18,3 +18,173 @@ describe("errors", () => {
         }).toThrowWithMessage(TypeError, "Not an object of type Temporal.Instant");
     });
 });
+
+describe("rounding modes", () => {
+    const earlier = new Temporal.Instant(
+        217178610_123_456_789n /* 1976-11-18T15:23:30.123456789Z */
+    );
+    const later = new Temporal.Instant(
+        1572345998_271_986_289n /* 2019-10-29T10:46:38.271986289Z */
+    );
+    const largestUnit = "hours";
+
+    test("'ceil' rounding mode", () => {
+        const expected = [
+            ["hours", "PT376436H", "-PT376435H"],
+            ["minutes", "PT376435H24M", "-PT376435H23M"],
+            ["seconds", "PT376435H23M9S", "-PT376435H23M8S"],
+            ["milliseconds", "PT376435H23M8.149S", "-PT376435H23M8.148S"],
+            ["microseconds", "PT376435H23M8.14853S", "-PT376435H23M8.148529S"],
+            ["nanoseconds", "PT376435H23M8.1485295S", "-PT376435H23M8.1485295S"],
+        ];
+
+        const roundingMode = "ceil";
+        expected.forEach(([smallestUnit, expectedPositive, expectedNegative]) => {
+            const untilPositive = earlier.until(later, { largestUnit, smallestUnit, roundingMode });
+            expect(untilPositive.toString()).toBe(expectedPositive);
+
+            const untilNegative = later.until(earlier, { largestUnit, smallestUnit, roundingMode });
+            expect(untilNegative.toString()).toBe(expectedNegative);
+        });
+    });
+
+    test("'expand' rounding mode", () => {
+        const expected = [
+            ["hours", "PT376436H", "-PT376436H"],
+            ["minutes", "PT376435H24M", "-PT376435H24M"],
+            ["seconds", "PT376435H23M9S", "-PT376435H23M9S"],
+            ["milliseconds", "PT376435H23M8.149S", "-PT376435H23M8.149S"],
+            ["microseconds", "PT376435H23M8.14853S", "-PT376435H23M8.14853S"],
+            ["nanoseconds", "PT376435H23M8.1485295S", "-PT376435H23M8.1485295S"],
+        ];
+
+        const roundingMode = "expand";
+        expected.forEach(([smallestUnit, expectedPositive, expectedNegative]) => {
+            const untilPositive = earlier.until(later, { largestUnit, smallestUnit, roundingMode });
+            expect(untilPositive.toString()).toBe(expectedPositive);
+
+            const untilNegative = later.until(earlier, { largestUnit, smallestUnit, roundingMode });
+            expect(untilNegative.toString()).toBe(expectedNegative);
+        });
+    });
+
+    test("'floor' rounding mode", () => {
+        const expected = [
+            ["hours", "PT376435H", "-PT376436H"],
+            ["minutes", "PT376435H23M", "-PT376435H24M"],
+            ["seconds", "PT376435H23M8S", "-PT376435H23M9S"],
+            ["milliseconds", "PT376435H23M8.148S", "-PT376435H23M8.149S"],
+            ["microseconds", "PT376435H23M8.148529S", "-PT376435H23M8.14853S"],
+            ["nanoseconds", "PT376435H23M8.1485295S", "-PT376435H23M8.1485295S"],
+        ];
+
+        const roundingMode = "floor";
+        expected.forEach(([smallestUnit, expectedPositive, expectedNegative]) => {
+            const untilPositive = earlier.until(later, { largestUnit, smallestUnit, roundingMode });
+            expect(untilPositive.toString()).toBe(expectedPositive);
+
+            const untilNegative = later.until(earlier, { largestUnit, smallestUnit, roundingMode });
+            expect(untilNegative.toString()).toBe(expectedNegative);
+        });
+    });
+
+    test("'halfCeil' rounding mode", () => {
+        const expected = [
+            ["hours", "PT376435H", "-PT376435H"],
+            ["minutes", "PT376435H23M", "-PT376435H23M"],
+            ["seconds", "PT376435H23M8S", "-PT376435H23M8S"],
+            ["milliseconds", "PT376435H23M8.149S", "-PT376435H23M8.149S"],
+            ["microseconds", "PT376435H23M8.14853S", "-PT376435H23M8.148529S"],
+            ["nanoseconds", "PT376435H23M8.1485295S", "-PT376435H23M8.1485295S"],
+        ];
+
+        const roundingMode = "halfCeil";
+        expected.forEach(([smallestUnit, expectedPositive, expectedNegative]) => {
+            const untilPositive = earlier.until(later, { largestUnit, smallestUnit, roundingMode });
+            expect(untilPositive.toString()).toBe(expectedPositive);
+
+            const untilNegative = later.until(earlier, { largestUnit, smallestUnit, roundingMode });
+            expect(untilNegative.toString()).toBe(expectedNegative);
+        });
+    });
+
+    test("'halfEven' rounding mode", () => {
+        const expected = [
+            ["hours", "PT376435H", "-PT376435H"],
+            ["minutes", "PT376435H23M", "-PT376435H23M"],
+            ["seconds", "PT376435H23M8S", "-PT376435H23M8S"],
+            ["milliseconds", "PT376435H23M8.149S", "-PT376435H23M8.149S"],
+            ["microseconds", "PT376435H23M8.14853S", "-PT376435H23M8.14853S"],
+            ["nanoseconds", "PT376435H23M8.1485295S", "-PT376435H23M8.1485295S"],
+        ];
+
+        const roundingMode = "halfEven";
+        expected.forEach(([smallestUnit, expectedPositive, expectedNegative]) => {
+            const untilPositive = earlier.until(later, { largestUnit, smallestUnit, roundingMode });
+            expect(untilPositive.toString()).toBe(expectedPositive);
+
+            const untilNegative = later.until(earlier, { largestUnit, smallestUnit, roundingMode });
+            expect(untilNegative.toString()).toBe(expectedNegative);
+        });
+    });
+
+    test("'halfExpand' rounding mode", () => {
+        const expected = [
+            ["hours", "PT376435H", "-PT376435H"],
+            ["minutes", "PT376435H23M", "-PT376435H23M"],
+            ["seconds", "PT376435H23M8S", "-PT376435H23M8S"],
+            ["milliseconds", "PT376435H23M8.149S", "-PT376435H23M8.149S"],
+            ["microseconds", "PT376435H23M8.14853S", "-PT376435H23M8.14853S"],
+            ["nanoseconds", "PT376435H23M8.1485295S", "-PT376435H23M8.1485295S"],
+        ];
+
+        const roundingMode = "halfExpand";
+        expected.forEach(([smallestUnit, expectedPositive, expectedNegative]) => {
+            const untilPositive = earlier.until(later, { largestUnit, smallestUnit, roundingMode });
+            expect(untilPositive.toString()).toBe(expectedPositive);
+
+            const untilNegative = later.until(earlier, { largestUnit, smallestUnit, roundingMode });
+            expect(untilNegative.toString()).toBe(expectedNegative);
+        });
+    });
+
+    test("'halfFloor' rounding mode", () => {
+        const expected = [
+            ["hours", "PT376435H", "-PT376435H"],
+            ["minutes", "PT376435H23M", "-PT376435H23M"],
+            ["seconds", "PT376435H23M8S", "-PT376435H23M8S"],
+            ["milliseconds", "PT376435H23M8.149S", "-PT376435H23M8.149S"],
+            ["microseconds", "PT376435H23M8.148529S", "-PT376435H23M8.14853S"],
+            ["nanoseconds", "PT376435H23M8.1485295S", "-PT376435H23M8.1485295S"],
+        ];
+
+        const roundingMode = "halfFloor";
+        expected.forEach(([smallestUnit, expectedPositive, expectedNegative]) => {
+            const untilPositive = earlier.until(later, { largestUnit, smallestUnit, roundingMode });
+            expect(untilPositive.toString()).toBe(expectedPositive);
+
+            const untilNegative = later.until(earlier, { largestUnit, smallestUnit, roundingMode });
+            expect(untilNegative.toString()).toBe(expectedNegative);
+        });
+    });
+
+    test("'halfTrunc' rounding mode", () => {
+        const expected = [
+            ["hours", "PT376435H", "-PT376435H"],
+            ["minutes", "PT376435H23M", "-PT376435H23M"],
+            ["seconds", "PT376435H23M8S", "-PT376435H23M8S"],
+            ["milliseconds", "PT376435H23M8.149S", "-PT376435H23M8.149S"],
+            ["microseconds", "PT376435H23M8.148529S", "-PT376435H23M8.148529S"],
+            ["nanoseconds", "PT376435H23M8.1485295S", "-PT376435H23M8.1485295S"],
+        ];
+
+        const roundingMode = "halfTrunc";
+        expected.forEach(([smallestUnit, expectedPositive, expectedNegative]) => {
+            const untilPositive = earlier.until(later, { largestUnit, smallestUnit, roundingMode });
+            expect(untilPositive.toString()).toBe(expectedPositive);
+
+            const untilNegative = later.until(earlier, { largestUnit, smallestUnit, roundingMode });
+            expect(untilNegative.toString()).toBe(expectedNegative);
+        });
+    });
+});
diff --git a/Userland/Libraries/LibJS/Tests/builtins/Temporal/ZonedDateTime/ZonedDateTime.prototype.since.js b/Userland/Libraries/LibJS/Tests/builtins/Temporal/ZonedDateTime/ZonedDateTime.prototype.since.js
index 6f3801d4d8..4f5252e0b1 100644
--- a/Userland/Libraries/LibJS/Tests/builtins/Temporal/ZonedDateTime/ZonedDateTime.prototype.since.js
+++ b/Userland/Libraries/LibJS/Tests/builtins/Temporal/ZonedDateTime/ZonedDateTime.prototype.since.js
@@ -113,3 +113,230 @@ describe("errors", () => {
         }).toThrowWithMessage(RangeError, "Cannot compare dates from two different time zones");
     });
 });
+
+describe("rounding modes", () => {
+    const earlier = new Temporal.ZonedDateTime(
+        1546935756_123_456_789n /* 2019-01-08T08:22:36.123456789+00:00 */,
+        "UTC"
+    );
+    const later = new Temporal.ZonedDateTime(
+        1631018380_987_654_289n /* 2021-09-07T12:39:40.987654289+00:00 */,
+        "UTC"
+    );
+
+    test("'ceil' rounding mode", () => {
+        const expected = [
+            ["years", "P3Y", "-P2Y"],
+            ["months", "P32M", "-P31M"],
+            ["weeks", "P140W", "-P139W"],
+            ["days", "P974D", "-P973D"],
+            ["hours", "PT23357H", "-PT23356H"],
+            ["minutes", "PT23356H18M", "-PT23356H17M"],
+            ["seconds", "PT23356H17M5S", "-PT23356H17M4S"],
+            ["milliseconds", "PT23356H17M4.865S", "-PT23356H17M4.864S"],
+            ["microseconds", "PT23356H17M4.864198S", "-PT23356H17M4.864197S"],
+            ["nanoseconds", "PT23356H17M4.8641975S", "-PT23356H17M4.8641975S"],
+        ];
+
+        const roundingMode = "ceil";
+        expected.forEach(([smallestUnit, expectedPositive, expectedNegative]) => {
+            const sincePositive = later.since(earlier, { smallestUnit, roundingMode });
+            expect(sincePositive.toString()).toBe(expectedPositive);
+
+            const sinceNegative = earlier.since(later, { smallestUnit, roundingMode });
+            expect(sinceNegative.toString()).toBe(expectedNegative);
+        });
+    });
+
+    test("'expand' rounding mode", () => {
+        const expected = [
+            ["years", "P3Y", "-P3Y"],
+            ["months", "P32M", "-P32M"],
+            ["weeks", "P140W", "-P140W"],
+            ["days", "P974D", "-P974D"],
+            ["hours", "PT23357H", "-PT23357H"],
+            ["minutes", "PT23356H18M", "-PT23356H18M"],
+            ["seconds", "PT23356H17M5S", "-PT23356H17M5S"],
+            ["milliseconds", "PT23356H17M4.865S", "-PT23356H17M4.865S"],
+            ["microseconds", "PT23356H17M4.864198S", "-PT23356H17M4.864198S"],
+            ["nanoseconds", "PT23356H17M4.8641975S", "-PT23356H17M4.8641975S"],
+        ];
+
+        const roundingMode = "expand";
+        expected.forEach(([smallestUnit, expectedPositive, expectedNegative]) => {
+            const sincePositive = later.since(earlier, { smallestUnit, roundingMode });
+            expect(sincePositive.toString()).toBe(expectedPositive);
+
+            const sinceNegative = earlier.since(later, { smallestUnit, roundingMode });
+            expect(sinceNegative.toString()).toBe(expectedNegative);
+        });
+    });
+
+    test("'floor' rounding mode", () => {
+        const expected = [
+            ["years", "P2Y", "-P3Y"],
+            ["months", "P31M", "-P32M"],
+            ["weeks", "P139W", "-P140W"],
+            ["days", "P973D", "-P974D"],
+            ["hours", "PT23356H", "-PT23357H"],
+            ["minutes", "PT23356H17M", "-PT23356H18M"],
+            ["seconds", "PT23356H17M4S", "-PT23356H17M5S"],
+            ["milliseconds", "PT23356H17M4.864S", "-PT23356H17M4.865S"],
+            ["microseconds", "PT23356H17M4.864197S", "-PT23356H17M4.864198S"],
+            ["nanoseconds", "PT23356H17M4.8641975S", "-PT23356H17M4.8641975S"],
+        ];
+
+        const roundingMode = "floor";
+        expected.forEach(([smallestUnit, expectedPositive, expectedNegative]) => {
+            const sincePositive = later.since(earlier, { smallestUnit, roundingMode });
+            expect(sincePositive.toString()).toBe(expectedPositive);
+
+            const sinceNegative = earlier.since(later, { smallestUnit, roundingMode });
+            expect(sinceNegative.toString()).toBe(expectedNegative);
+        });
+    });
+
+    test("'trunc' rounding mode", () => {
+        const expected = [
+            ["years", "P2Y", "-P2Y"],
+            ["months", "P31M", "-P31M"],
+            ["weeks", "P139W", "-P139W"],
+            ["days", "P973D", "-P973D"],
+            ["hours", "PT23356H", "-PT23356H"],
+            ["minutes", "PT23356H17M", "-PT23356H17M"],
+            ["seconds", "PT23356H17M4S", "-PT23356H17M4S"],
+            ["milliseconds", "PT23356H17M4.864S", "-PT23356H17M4.864S"],
+            ["microseconds", "PT23356H17M4.864197S", "-PT23356H17M4.864197S"],
+            ["nanoseconds", "PT23356H17M4.8641975S", "-PT23356H17M4.8641975S"],
+        ];
+
+        const roundingMode = "trunc";
+        expected.forEach(([smallestUnit, expectedPositive, expectedNegative]) => {
+            const sincePositive = later.since(earlier, { smallestUnit, roundingMode });
+            expect(sincePositive.toString()).toBe(expectedPositive);
+
+            const sinceNegative = earlier.since(later, { smallestUnit, roundingMode });
+            expect(sinceNegative.toString()).toBe(expectedNegative);
+        });
+    });
+
+    test("'halfCeil' rounding mode", () => {
+        const expected = [
+            ["years", "P3Y", "-P3Y"],
+            ["months", "P32M", "-P32M"],
+            ["weeks", "P139W", "-P139W"],
+            ["days", "P973D", "-P973D"],
+            ["hours", "PT23356H", "-PT23356H"],
+            ["minutes", "PT23356H17M", "-PT23356H17M"],
+            ["seconds", "PT23356H17M5S", "-PT23356H17M5S"],
+            ["milliseconds", "PT23356H17M4.864S", "-PT23356H17M4.864S"],
+            ["microseconds", "PT23356H17M4.864198S", "-PT23356H17M4.864197S"],
+            ["nanoseconds", "PT23356H17M4.8641975S", "-PT23356H17M4.8641975S"],
+        ];
+
+        const roundingMode = "halfCeil";
+        expected.forEach(([smallestUnit, expectedPositive, expectedNegative]) => {
+            const sincePositive = later.since(earlier, { smallestUnit, roundingMode });
+            expect(sincePositive.toString()).toBe(expectedPositive);
+
+            const sinceNegative = earlier.since(later, { smallestUnit, roundingMode });
+            expect(sinceNegative.toString()).toBe(expectedNegative);
+        });
+    });
+
+    test("'halfEven' rounding mode", () => {
+        const expected = [
+            ["years", "P3Y", "-P3Y"],
+            ["months", "P32M", "-P32M"],
+            ["weeks", "P139W", "-P139W"],
+            ["days", "P973D", "-P973D"],
+            ["hours", "PT23356H", "-PT23356H"],
+            ["minutes", "PT23356H17M", "-PT23356H17M"],
+            ["seconds", "PT23356H17M5S", "-PT23356H17M5S"],
+            ["milliseconds", "PT23356H17M4.864S", "-PT23356H17M4.864S"],
+            ["microseconds", "PT23356H17M4.864198S", "-PT23356H17M4.864198S"],
+            ["nanoseconds", "PT23356H17M4.8641975S", "-PT23356H17M4.8641975S"],
+        ];
+
+        const roundingMode = "halfEven";
+        expected.forEach(([smallestUnit, expectedPositive, expectedNegative]) => {
+            const sincePositive = later.since(earlier, { smallestUnit, roundingMode });
+            expect(sincePositive.toString()).toBe(expectedPositive);
+
+            const sinceNegative = earlier.since(later, { smallestUnit, roundingMode });
+            expect(sinceNegative.toString()).toBe(expectedNegative);
+        });
+    });
+
+    test("'halfTrunc' rounding mode", () => {
+        const expected = [
+            ["years", "P3Y", "-P3Y"],
+            ["months", "P32M", "-P32M"],
+            ["weeks", "P139W", "-P139W"],
+            ["days", "P973D", "-P973D"],
+            ["hours", "PT23356H", "-PT23356H"],
+            ["minutes", "PT23356H17M", "-PT23356H17M"],
+            ["seconds", "PT23356H17M5S", "-PT23356H17M5S"],
+            ["milliseconds", "PT23356H17M4.864S", "-PT23356H17M4.864S"],
+            ["microseconds", "PT23356H17M4.864197S", "-PT23356H17M4.864197S"],
+            ["nanoseconds", "PT23356H17M4.8641975S", "-PT23356H17M4.8641975S"],
+        ];
+
+        const roundingMode = "halfTrunc";
+        expected.forEach(([smallestUnit, expectedPositive, expectedNegative]) => {
+            const sincePositive = later.since(earlier, { smallestUnit, roundingMode });
+            expect(sincePositive.toString()).toBe(expectedPositive);
+
+            const sinceNegative = earlier.since(later, { smallestUnit, roundingMode });
+            expect(sinceNegative.toString()).toBe(expectedNegative);
+        });
+    });
+
+    test("'halfExpand' rounding mode", () => {
+        const expected = [
+            ["years", "P3Y", "-P3Y"],
+            ["months", "P32M", "-P32M"],
+            ["weeks", "P139W", "-P139W"],
+            ["days", "P973D", "-P973D"],
+            ["hours", "PT23356H", "-PT23356H"],
+            ["minutes", "PT23356H17M", "-PT23356H17M"],
+            ["seconds", "PT23356H17M5S", "-PT23356H17M5S"],
+            ["milliseconds", "PT23356H17M4.864S", "-PT23356H17M4.864S"],
+            ["microseconds", "PT23356H17M4.864198S", "-PT23356H17M4.864198S"],
+            ["nanoseconds", "PT23356H17M4.8641975S", "-PT23356H17M4.8641975S"],
+        ];
+
+        const roundingMode = "halfExpand";
+        expected.forEach(([smallestUnit, expectedPositive, expectedNegative]) => {
+            const sincePositive = later.since(earlier, { smallestUnit, roundingMode });
+            expect(sincePositive.toString()).toBe(expectedPositive);
+
+            const sinceNegative = earlier.since(later, { smallestUnit, roundingMode });
+            expect(sinceNegative.toString()).toBe(expectedNegative);
+        });
+    });
+
+    test("'halfFloor' rounding mode", () => {
+        const expected = [
+            ["years", "P3Y", "-P3Y"],
+            ["months", "P32M", "-P32M"],
+            ["weeks", "P139W", "-P139W"],
+            ["days", "P973D", "-P973D"],
+            ["hours", "PT23356H", "-PT23356H"],
+            ["minutes", "PT23356H17M", "-PT23356H17M"],
+            ["seconds", "PT23356H17M5S", "-PT23356H17M5S"],
+            ["milliseconds", "PT23356H17M4.864S", "-PT23356H17M4.864S"],
+            ["microseconds", "PT23356H17M4.864197S", "-PT23356H17M4.864198S"],
+            ["nanoseconds", "PT23356H17M4.8641975S", "-PT23356H17M4.8641975S"],
+        ];
+
+        const roundingMode = "halfFloor";
+        expected.forEach(([smallestUnit, expectedPositive, expectedNegative]) => {
+            const sincePositive = later.since(earlier, { smallestUnit, roundingMode });
+            expect(sincePositive.toString()).toBe(expectedPositive);
+
+            const sinceNegative = earlier.since(later, { smallestUnit, roundingMode });
+            expect(sinceNegative.toString()).toBe(expectedNegative);
+        });
+    });
+});
diff --git a/Userland/Libraries/LibJS/Tests/builtins/Temporal/ZonedDateTime/ZonedDateTime.prototype.until.js b/Userland/Libraries/LibJS/Tests/builtins/Temporal/ZonedDateTime/ZonedDateTime.prototype.until.js
index 8e8fd0d364..d7c2658fab 100644
--- a/Userland/Libraries/LibJS/Tests/builtins/Temporal/ZonedDateTime/ZonedDateTime.prototype.until.js
+++ b/Userland/Libraries/LibJS/Tests/builtins/Temporal/ZonedDateTime/ZonedDateTime.prototype.until.js
@@ -113,3 +113,230 @@ describe("errors", () => {
         }).toThrowWithMessage(RangeError, "Cannot compare dates from two different time zones");
     });
 });
+
+describe("rounding modes", () => {
+    const earlier = new Temporal.ZonedDateTime(
+        1546935756_123_456_789n /* 2019-01-08T08:22:36.123456789+00:00 */,
+        "UTC"
+    );
+    const later = new Temporal.ZonedDateTime(
+        1631018380_987_654_289n /* 2021-09-07T12:39:40.987654289+00:00 */,
+        "UTC"
+    );
+
+    test("'ceil' rounding mode", () => {
+        const expected = [
+            ["years", "P3Y", "-P2Y"],
+            ["months", "P32M", "-P31M"],
+            ["weeks", "P140W", "-P139W"],
+            ["days", "P974D", "-P973D"],
+            ["hours", "PT23357H", "-PT23356H"],
+            ["minutes", "PT23356H18M", "-PT23356H17M"],
+            ["seconds", "PT23356H17M5S", "-PT23356H17M4S"],
+            ["milliseconds", "PT23356H17M4.865S", "-PT23356H17M4.864S"],
+            ["microseconds", "PT23356H17M4.864198S", "-PT23356H17M4.864197S"],
+            ["nanoseconds", "PT23356H17M4.8641975S", "-PT23356H17M4.8641975S"],
+        ];
+
+        const roundingMode = "ceil";
+        expected.forEach(([smallestUnit, expectedPositive, expectedNegative]) => {
+            const untilPositive = earlier.until(later, { smallestUnit, roundingMode });
+            expect(untilPositive.toString()).toBe(expectedPositive);
+
+            const untilNegative = later.until(earlier, { smallestUnit, roundingMode });
+            expect(untilNegative.toString()).toBe(expectedNegative);
+        });
+    });
+
+    test("'expand' rounding mode", () => {
+        const expected = [
+            ["years", "P3Y", "-P3Y"],
+            ["months", "P32M", "-P32M"],
+            ["weeks", "P140W", "-P140W"],
+            ["days", "P974D", "-P974D"],
+            ["hours", "PT23357H", "-PT23357H"],
+            ["minutes", "PT23356H18M", "-PT23356H18M"],
+            ["seconds", "PT23356H17M5S", "-PT23356H17M5S"],
+            ["milliseconds", "PT23356H17M4.865S", "-PT23356H17M4.865S"],
+            ["microseconds", "PT23356H17M4.864198S", "-PT23356H17M4.864198S"],
+            ["nanoseconds", "PT23356H17M4.8641975S", "-PT23356H17M4.8641975S"],
+        ];
+
+        const roundingMode = "expand";
+        expected.forEach(([smallestUnit, expectedPositive, expectedNegative]) => {
+            const untilPositive = earlier.until(later, { smallestUnit, roundingMode });
+            expect(untilPositive.toString()).toBe(expectedPositive);
+
+            const untilNegative = later.until(earlier, { smallestUnit, roundingMode });
+            expect(untilNegative.toString()).toBe(expectedNegative);
+        });
+    });
+
+    test("'floor' rounding mode", () => {
+        const expected = [
+            ["years", "P2Y", "-P3Y"],
+            ["months", "P31M", "-P32M"],
+            ["weeks", "P139W", "-P140W"],
+            ["days", "P973D", "-P974D"],
+            ["hours", "PT23356H", "-PT23357H"],
+            ["minutes", "PT23356H17M", "-PT23356H18M"],
+            ["seconds", "PT23356H17M4S", "-PT23356H17M5S"],
+            ["milliseconds", "PT23356H17M4.864S", "-PT23356H17M4.865S"],
+            ["microseconds", "PT23356H17M4.864197S", "-PT23356H17M4.864198S"],
+            ["nanoseconds", "PT23356H17M4.8641975S", "-PT23356H17M4.8641975S"],
+        ];
+
+        const roundingMode = "floor";
+        expected.forEach(([smallestUnit, expectedPositive, expectedNegative]) => {
+            const untilPositive = earlier.until(later, { smallestUnit, roundingMode });
+            expect(untilPositive.toString()).toBe(expectedPositive);
+
+            const untilNegative = later.until(earlier, { smallestUnit, roundingMode });
+            expect(untilNegative.toString()).toBe(expectedNegative);
+        });
+    });
+
+    test("'trunc' rounding mode", () => {
+        const expected = [
+            ["years", "P2Y", "-P2Y"],
+            ["months", "P31M", "-P31M"],
+            ["weeks", "P139W", "-P139W"],
+            ["days", "P973D", "-P973D"],
+            ["hours", "PT23356H", "-PT23356H"],
+            ["minutes", "PT23356H17M", "-PT23356H17M"],
+            ["seconds", "PT23356H17M4S", "-PT23356H17M4S"],
+            ["milliseconds", "PT23356H17M4.864S", "-PT23356H17M4.864S"],
+            ["microseconds", "PT23356H17M4.864197S", "-PT23356H17M4.864197S"],
+            ["nanoseconds", "PT23356H17M4.8641975S", "-PT23356H17M4.8641975S"],
+        ];
+
+        const roundingMode = "trunc";
+        expected.forEach(([smallestUnit, expectedPositive, expectedNegative]) => {
+            const untilPositive = earlier.until(later, { smallestUnit, roundingMode });
+            expect(untilPositive.toString()).toBe(expectedPositive);
+
+            const untilNegative = later.until(earlier, { smallestUnit, roundingMode });
+            expect(untilNegative.toString()).toBe(expectedNegative);
+        });
+    });
+
+    test("'halfCeil' rounding mode", () => {
+        const expected = [
+            ["years", "P3Y", "-P3Y"],
+            ["months", "P32M", "-P32M"],
+            ["weeks", "P139W", "-P139W"],
+            ["days", "P973D", "-P973D"],
+            ["hours", "PT23356H", "-PT23356H"],
+            ["minutes", "PT23356H17M", "-PT23356H17M"],
+            ["seconds", "PT23356H17M5S", "-PT23356H17M5S"],
+            ["milliseconds", "PT23356H17M4.864S", "-PT23356H17M4.864S"],
+            ["microseconds", "PT23356H17M4.864198S", "-PT23356H17M4.864197S"],
+            ["nanoseconds", "PT23356H17M4.8641975S", "-PT23356H17M4.8641975S"],
+        ];
+
+        const roundingMode = "halfCeil";
+        expected.forEach(([smallestUnit, expectedPositive, expectedNegative]) => {
+            const untilPositive = earlier.until(later, { smallestUnit, roundingMode });
+            expect(untilPositive.toString()).toBe(expectedPositive);
+
+            const untilNegative = later.until(earlier, { smallestUnit, roundingMode });
+            expect(untilNegative.toString()).toBe(expectedNegative);
+        });
+    });
+
+    test("'halfEven' rounding mode", () => {
+        const expected = [
+            ["years", "P3Y", "-P3Y"],
+            ["months", "P32M", "-P32M"],
+            ["weeks", "P139W", "-P139W"],
+            ["days", "P973D", "-P973D"],
+            ["hours", "PT23356H", "-PT23356H"],
+            ["minutes", "PT23356H17M", "-PT23356H17M"],
+            ["seconds", "PT23356H17M5S", "-PT23356H17M5S"],
+            ["milliseconds", "PT23356H17M4.864S", "-PT23356H17M4.864S"],
+            ["microseconds", "PT23356H17M4.864198S", "-PT23356H17M4.864198S"],
+            ["nanoseconds", "PT23356H17M4.8641975S", "-PT23356H17M4.8641975S"],
+        ];
+
+        const roundingMode = "halfEven";
+        expected.forEach(([smallestUnit, expectedPositive, expectedNegative]) => {
+            const untilPositive = earlier.until(later, { smallestUnit, roundingMode });
+            expect(untilPositive.toString()).toBe(expectedPositive);
+
+            const untilNegative = later.until(earlier, { smallestUnit, roundingMode });
+            expect(untilNegative.toString()).toBe(expectedNegative);
+        });
+    });
+
+    test("'halfTrunc' rounding mode", () => {
+        const expected = [
+            ["years", "P3Y", "-P3Y"],
+            ["months", "P32M", "-P32M"],
+            ["weeks", "P139W", "-P139W"],
+            ["days", "P973D", "-P973D"],
+            ["hours", "PT23356H", "-PT23356H"],
+            ["minutes", "PT23356H17M", "-PT23356H17M"],
+            ["seconds", "PT23356H17M5S", "-PT23356H17M5S"],
+            ["milliseconds", "PT23356H17M4.864S", "-PT23356H17M4.864S"],
+            ["microseconds", "PT23356H17M4.864197S", "-PT23356H17M4.864197S"],
+            ["nanoseconds", "PT23356H17M4.8641975S", "-PT23356H17M4.8641975S"],
+        ];
+
+        const roundingMode = "halfTrunc";
+        expected.forEach(([smallestUnit, expectedPositive, expectedNegative]) => {
+            const untilPositive = earlier.until(later, { smallestUnit, roundingMode });
+            expect(untilPositive.toString()).toBe(expectedPositive);
+
+            const untilNegative = later.until(earlier, { smallestUnit, roundingMode });
+            expect(untilNegative.toString()).toBe(expectedNegative);
+        });
+    });
+
+    test("'halfExpand' rounding mode", () => {
+        const expected = [
+            ["years", "P3Y", "-P3Y"],
+            ["months", "P32M", "-P32M"],
+            ["weeks", "P139W", "-P139W"],
+            ["days", "P973D", "-P973D"],
+            ["hours", "PT23356H", "-PT23356H"],
+            ["minutes", "PT23356H17M", "-PT23356H17M"],
+            ["seconds", "PT23356H17M5S", "-PT23356H17M5S"],
+            ["milliseconds", "PT23356H17M4.864S", "-PT23356H17M4.864S"],
+            ["microseconds", "PT23356H17M4.864198S", "-PT23356H17M4.864198S"],
+            ["nanoseconds", "PT23356H17M4.8641975S", "-PT23356H17M4.8641975S"],
+        ];
+
+        const roundingMode = "halfExpand";
+        expected.forEach(([smallestUnit, expectedPositive, expectedNegative]) => {
+            const untilPositive = earlier.until(later, { smallestUnit, roundingMode });
+            expect(untilPositive.toString()).toBe(expectedPositive);
+
+            const untilNegative = later.until(earlier, { smallestUnit, roundingMode });
+            expect(untilNegative.toString()).toBe(expectedNegative);
+        });
+    });
+
+    test("'halfFloor' rounding mode", () => {
+        const expected = [
+            ["years", "P3Y", "-P3Y"],
+            ["months", "P32M", "-P32M"],
+            ["weeks", "P139W", "-P139W"],
+            ["days", "P973D", "-P973D"],
+            ["hours", "PT23356H", "-PT23356H"],
+            ["minutes", "PT23356H17M", "-PT23356H17M"],
+            ["seconds", "PT23356H17M5S", "-PT23356H17M5S"],
+            ["milliseconds", "PT23356H17M4.864S", "-PT23356H17M4.864S"],
+            ["microseconds", "PT23356H17M4.864197S", "-PT23356H17M4.864198S"],
+            ["nanoseconds", "PT23356H17M4.8641975S", "-PT23356H17M4.8641975S"],
+        ];
+
+        const roundingMode = "halfFloor";
+        expected.forEach(([smallestUnit, expectedPositive, expectedNegative]) => {
+            const untilPositive = earlier.until(later, { smallestUnit, roundingMode });
+            expect(untilPositive.toString()).toBe(expectedPositive);
+
+            const untilNegative = later.until(earlier, { smallestUnit, roundingMode });
+            expect(untilNegative.toString()).toBe(expectedNegative);
+        });
+    });
+});