diff --git a/Userland/Libraries/LibC/time.cpp b/Userland/Libraries/LibC/time.cpp
index c1df69c6a5..a1591c8051 100644
--- a/Userland/Libraries/LibC/time.cpp
+++ b/Userland/Libraries/LibC/time.cpp
@@ -112,13 +112,18 @@ static bool is_valid_time(time_t timestamp)
     return (timestamp >= smallest_possible_time) && (timestamp <= biggest_possible_time);
 }
 
-static struct tm* time_to_tm(struct tm* tm, time_t t)
+static struct tm* time_to_tm(struct tm* tm, time_t t, StringView time_zone)
 {
     if (!is_valid_time(t)) {
         errno = EOVERFLOW;
         return nullptr;
     }
 
+    if (auto offset = TimeZone::get_time_zone_offset(time_zone, AK::Time::from_seconds(t)); offset.has_value()) {
+        tm->tm_isdst = offset->in_dst == TimeZone::InDST::Yes;
+        t += offset->seconds;
+    }
+
     int year = 1970;
     for (; t >= days_in_year(year) * __seconds_per_day; ++year)
         t -= days_in_year(year) * __seconds_per_day;
@@ -146,7 +151,7 @@ static struct tm* time_to_tm(struct tm* tm, time_t t)
     return tm;
 }
 
-static time_t tm_to_time(struct tm* tm, long timezone_adjust_seconds)
+static time_t tm_to_time(struct tm* tm, StringView time_zone)
 {
     // "The original values of the tm_wday and tm_yday components of the structure are ignored,
     // and the original values of the other components are not restricted to the ranges described in <time.h>.
@@ -156,8 +161,6 @@ static time_t tm_to_time(struct tm* tm, long timezone_adjust_seconds)
     // but with their values forced to the ranges indicated in the <time.h> entry;
     // the final value of tm_mday shall not be set until tm_mon and tm_year are determined."
 
-    // FIXME: Handle tm_isdst eventually.
-
     tm->tm_year += tm->tm_mon / 12;
     tm->tm_mon %= 12;
     if (tm->tm_mon < 0) {
@@ -167,7 +170,17 @@ static time_t tm_to_time(struct tm* tm, long timezone_adjust_seconds)
 
     tm->tm_yday = day_of_year(1900 + tm->tm_year, tm->tm_mon + 1, tm->tm_mday);
     time_t days_since_epoch = years_to_days_since_epoch(1900 + tm->tm_year) + tm->tm_yday;
-    auto timestamp = ((days_since_epoch * 24 + tm->tm_hour) * 60 + tm->tm_min) * 60 + tm->tm_sec + timezone_adjust_seconds;
+    auto timestamp = ((days_since_epoch * 24 + tm->tm_hour) * 60 + tm->tm_min) * 60 + tm->tm_sec;
+
+    if (tm->tm_isdst < 0) {
+        if (auto offset = TimeZone::get_time_zone_offset(time_zone, AK::Time::from_seconds(timestamp)); offset.has_value())
+            timestamp -= offset->seconds;
+    } else {
+        auto index = tm->tm_isdst == 0 ? 0 : 1;
+
+        if (auto offsets = TimeZone::get_named_time_zone_offsets(time_zone, AK::Time::from_seconds(timestamp)); offsets.has_value())
+            timestamp -= offsets->at(index).seconds;
+    }
 
     if (!is_valid_time(timestamp)) {
         errno = EOVERFLOW;
@@ -180,7 +193,7 @@ static time_t tm_to_time(struct tm* tm, long timezone_adjust_seconds)
 time_t mktime(struct tm* tm)
 {
     tzset();
-    return tm_to_time(tm, daylight ? altzone : timezone);
+    return tm_to_time(tm, __tzname);
 }
 
 struct tm* localtime(time_t const* t)
@@ -196,12 +209,12 @@ struct tm* localtime_r(time_t const* t, struct tm* tm)
     if (!t)
         return nullptr;
 
-    return time_to_tm(tm, *t - (daylight ? altzone : timezone));
+    return time_to_tm(tm, *t, __tzname);
 }
 
 time_t timegm(struct tm* tm)
 {
-    return tm_to_time(tm, 0);
+    return tm_to_time(tm, { __utc, __builtin_strlen(__utc) });
 }
 
 struct tm* gmtime(time_t const* t)
@@ -214,7 +227,7 @@ struct tm* gmtime_r(time_t const* t, struct tm* tm)
 {
     if (!t)
         return nullptr;
-    return time_to_tm(tm, *t);
+    return time_to_tm(tm, *t, { __utc, __builtin_strlen(__utc) });
 }
 
 char* asctime(const struct tm* tm)
diff --git a/Userland/Libraries/LibCore/DateTime.cpp b/Userland/Libraries/LibCore/DateTime.cpp
index bf09253f77..350e076658 100644
--- a/Userland/Libraries/LibCore/DateTime.cpp
+++ b/Userland/Libraries/LibCore/DateTime.cpp
@@ -92,13 +92,13 @@ String DateTime::to_string(StringView format) const
     int const format_len = format.length();
 
     auto format_time_zone_offset = [&](bool with_separator) {
-#if defined(__serenity__)
-        auto offset_seconds = daylight ? -altzone : -timezone;
-#elif !defined(__FreeBSD__)
-        auto offset_seconds = -timezone;
-#else
-        auto offset_seconds = 0;
-#endif
+        struct tm gmt_tm;
+        gmtime_r(&m_timestamp, &gmt_tm);
+
+        gmt_tm.tm_isdst = -1;
+        auto gmt_timestamp = mktime(&gmt_tm);
+
+        auto offset_seconds = static_cast<time_t>(difftime(m_timestamp, gmt_timestamp));
         StringView offset_sign;
 
         if (offset_seconds >= 0) {
@@ -251,11 +251,7 @@ String DateTime::to_string(StringView format) const
                 format_time_zone_offset(true);
                 break;
             case 'Z': {
-#ifndef __FreeBSD__
-                auto const* timezone_name = tzname[daylight];
-#else
-                auto const* timezone_name = tzname[0];
-#endif
+                auto const* timezone_name = tzname[tm.tm_isdst == 0 ? 0 : 1];
                 builder.append({ timezone_name, strlen(timezone_name) });
                 break;
             }