time/Zone: add native Windows implementation

src/time/Zone.cxx

@@ -32,19 +32,35 @@
 
 #include "Zone.hxx"
 
+#ifdef _WIN32
+#include <profileapi.h>
+#include <sysinfoapi.h>
+#include <timezoneapi.h>
+#else
 #include <time.h>
+#endif
 
 int
 GetTimeZoneOffset() noexcept
 {
-	time_t t = 1234567890;
 #ifdef _WIN32
-	struct tm *p = gmtime(&t);
+	TIME_ZONE_INFORMATION TimeZoneInformation;
+	DWORD tzi = GetTimeZoneInformation(&TimeZoneInformation);
+
+	int offset = -TimeZoneInformation.Bias * 60;
+	if (tzi == TIME_ZONE_ID_STANDARD)
+		offset -= TimeZoneInformation.StandardBias * 60;
+
+	if (tzi == TIME_ZONE_ID_DAYLIGHT)
+		offset -= TimeZoneInformation.DaylightBias * 60;
+
+	return offset;
 #else
+	time_t t = 1234567890;
 	struct tm tm;
 	tm.tm_isdst = 0;
 	struct tm *p = &tm;
 	gmtime_r(&t, p);
-#endif
 	return t - mktime(p);
+#endif
 }