LibC+LibCore: Use tm_isdst to handle time zone offsets in DST

Previously, we were incorrectly assuming that the daylight global
variable indicated whether the current time zone is in DST. In reality,
the daylight variable only indicates whether a time zone *can* be in
DST.

Instead, the tm structure has a tm_isdst member that should be used for
this purpose. Ensure our LibC handles tm_isdst, and avoid errant usage
of the daylight variable in Core::DateTime.

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)

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;
             }