TimeZone: delegate to ICU on Windows

Unfortunately, Windows does not have full time zone information
available.  For many cases we can make do with the system information,
augmented with alternate names (the Olson-to-Windows conversion
database).  This papers over the inability to deserialize the full
information in the registry by falling back to ICU to get the
information for the DST time conversion.

Author: compnerd

CoreFoundation/NumberDate.subproj/CFTimeZone.c

@@ -1679,18 +1679,34 @@ CFTimeInterval CFTimeZoneGetDaylightSavingTimeOffset(CFTimeZoneRef tz, CFAbsolut
     return 0.0;
 }
 
+extern UCalendar *__CFCalendarCreateUCalendar(CFStringRef calendarID, CFStringRef localeID, CFTimeZoneRef tz);
+
 CFAbsoluteTime CFTimeZoneGetNextDaylightSavingTimeTransition(CFTimeZoneRef tz, CFAbsoluteTime at) {
     CF_OBJC_FUNCDISPATCHV(CFTimeZoneGetTypeID(), CFTimeInterval, (NSTimeZone *)tz, _nextDaylightSavingTimeTransitionAfterAbsoluteTime:at);
     __CFGenericValidateType(tz, CFTimeZoneGetTypeID());
+#if TARGET_OS_WIN32
+    UErrorCode status = U_ZERO_ERROR;
+    UCalendar *ucal = __CFCalendarCreateUCalendar(NULL, CFSTR("C"), tz);
+    if (ucal == NULL) {
+      return 0.0;
+    }
+    ucal_setMillis(ucal, (at + kCFAbsoluteTimeIntervalSince1970) * 1000.0, &status);
+
+    UDate date;
+    ucal_getTimeZoneTransitionDate(ucal, UCAL_TZ_TRANSITION_NEXT, &date, &status);
+
+    ucal_close(ucal);
+
+    return (date / 1000.0) - kCFAbsoluteTimeIntervalSince1970;
+#else
     CFIndex idx = __CFBSearchTZPeriods(tz, at);
     if (tz->_periodCnt <= idx + 1) {
         return 0.0;
     }
     return (CFAbsoluteTime)__CFTZPeriodStartSeconds(&(tz->_periods[idx + 1]));
+#endif
 }
 
-extern UCalendar *__CFCalendarCreateUCalendar(CFStringRef calendarID, CFStringRef localeID, CFTimeZoneRef tz);
-
 #define BUFFER_SIZE 768
 
 CFStringRef CFTimeZoneCopyLocalizedName(CFTimeZoneRef tz, CFTimeZoneNameStyle style, CFLocaleRef locale) {