fix: combine date and time to convert loc datetime

Normally DST begins at A.M. 3 or 4. If we re-use conversion operator
of local_date and local_time independently, the conversion fails if
it is the day when DST begins or ends. Since local_date considers the
time is 00:00 A.M. and local_time does not consider DST because it
does not have any date information. We need to consider both date and
time information at the same time to convert it correctly.

toml/datetime.hpp

@@ -398,10 +398,31 @@ struct local_datetime
     {
         using internal_duration =
             typename std::chrono::system_clock::time_point::duration;
+
+        // Normally DST begins at A.M. 3 or 4. If we re-use conversion operator
+        // of local_date and local_time independently, the conversion fails if
+        // it is the day when DST begins or ends. Since local_date considers the
+        // time is 00:00 A.M. and local_time does not consider DST because it
+        // does not have any date information. We need to consider both date and
+        // time information at the same time to convert it correctly.
+
+        std::tm t;
+        t.tm_sec   = static_cast<int>(this->time.second);
+        t.tm_min   = static_cast<int>(this->time.minute);
+        t.tm_hour  = static_cast<int>(this->time.hour);
+        t.tm_mday  = static_cast<int>(this->date.day);
+        t.tm_mon   = static_cast<int>(this->date.month);
+        t.tm_year  = static_cast<int>(this->date.year) - 1900;
+        t.tm_wday  = 0; // the value will be ignored
+        t.tm_yday  = 0; // the value will be ignored
+        t.tm_isdst = -1;
+
         // std::mktime returns date as local time zone. no conversion needed
-        auto dt = std::chrono::system_clock::time_point(this->date);
+        auto dt = std::chrono::system_clock::from_time_t(std::mktime(&t));
         dt += std::chrono::duration_cast<internal_duration>(
-                   std::chrono::nanoseconds(this->time));
+                std::chrono::milliseconds(this->time.millisecond) +
+                std::chrono::microseconds(this->time.microsecond) +
+                std::chrono::nanoseconds (this->time.nanosecond));
         return dt;
     }