diff --git a/Libraries/LibC/time.cpp b/Libraries/LibC/time.cpp
index 2d22f8c22c..634bd21621 100644
--- a/Libraries/LibC/time.cpp
+++ b/Libraries/LibC/time.cpp
@@ -93,21 +93,28 @@ static void time_to_tm(struct tm* tm, time_t t)
     tm->tm_mday += days;
 }
 
-static time_t tm_to_time(struct tm* tm, long timezone_adjust)
+static time_t tm_to_time(struct tm* tm, long timezone_adjust_seconds)
 {
     int days = 0;
-    int seconds = tm->tm_hour * 3600 + tm->tm_min * 60 + tm->tm_sec;
-    for (int year = 70; year < tm->tm_year; ++year)
-        days += 365 + __is_leap_year(1900 + year);
+    if (tm->tm_year >= 70) {
+        for (int year = 70; year < tm->tm_year; ++year)
+            days += 365 + __is_leap_year(1900 + year);
+    } else {
+        for (int year = tm->tm_year; year < 70; ++year)
+            days -= 365 + __is_leap_year(1900 + year);
+    }
 
     tm->tm_yday = tm->tm_mday - 1;
+    // FIXME: What if tm->tm_mon < 0 or tm->tm_mon > 12?
     for (int month = 0; month < tm->tm_mon; ++month)
         tm->tm_yday += __days_per_month[month];
     if (tm->tm_mon > 1 && __is_leap_year(1900 + tm->tm_year))
         ++tm->tm_yday;
 
     days += tm->tm_yday;
-    return days * __seconds_per_day + seconds + timezone_adjust;
+
+    int seconds = tm->tm_hour * 3600 + tm->tm_min * 60 + tm->tm_sec;
+    return static_cast<time_t>(days) * __seconds_per_day + seconds + timezone_adjust_seconds;
 }
 
 time_t mktime(struct tm* tm)
diff --git a/Libraries/LibJS/Tests/builtins/Date/Date.UTC.js b/Libraries/LibJS/Tests/builtins/Date/Date.UTC.js
index 171e537209..8a5c0e09dc 100644
--- a/Libraries/LibJS/Tests/builtins/Date/Date.UTC.js
+++ b/Libraries/LibJS/Tests/builtins/Date/Date.UTC.js
@@ -1,5 +1,4 @@
 test("basic functionality", () => {
-    // FIXME: Years before 1970 don't work. Once they do, add a test. Also add a test with a year before 1900 then.
     expect(Date.UTC(2020)).toBe(1577836800000);
     expect(Date.UTC(2000, 10)).toBe(973036800000);
     expect(Date.UTC(1980, 5, 30)).toBe(331171200000);
@@ -7,4 +6,22 @@ test("basic functionality", () => {
     expect(Date.UTC(1970, 5, 30, 13, 30)).toBe(15600600000);
     expect(Date.UTC(1970, 0, 1, 0, 0, 59)).toBe(59000);
     expect(Date.UTC(1970, 0, 1, 0, 0, 0, 999)).toBe(999);
+
+    expect(Date.UTC(1969, 11, 31, 23, 59, 59, 817)).toBe(-183);
+
+    expect(Date.UTC(1799, 0)).toBe(-5396198400000);
+    expect(Date.UTC(1800, 0)).toBe(-5364662400000);
+    expect(Date.UTC(1801, 0)).toBe(-5333126400000);
+    expect(Date.UTC(1802, 0)).toBe(-5301590400000);
+    expect(Date.UTC(1803, 0)).toBe(-5270054400000);
+    expect(Date.UTC(1804, 0)).toBe(-5238518400000);
+
+    expect(Date.UTC(1999, 0)).toBe(915148800000);
+    expect(Date.UTC(2000, 0)).toBe(946684800000);
+    expect(Date.UTC(2001, 0)).toBe(978307200000);
+    expect(Date.UTC(2002, 0)).toBe(1009843200000);
+    expect(Date.UTC(2003, 0)).toBe(1041379200000);
+    expect(Date.UTC(2004, 0)).toBe(1072915200000);
+
+    expect(Date.UTC(20000, 0)).toBe(568971820800000);
 });