Fix: xpDateTime_to_time() would convert an xpDateTime_t which was in UTC

to the local timezone equivalent (not adjusting for offset). Now using timegm()
(aka mkgmtime()) to fix this scenario. Times in zones other than the local time
zone or UTC are explicitly not supported by this function now (returns
INVALID_TIME in that scenario).

src/xpdev/xpdatetime.c

@@ -8,7 +8,7 @@
  * @format.tab-size 4		(Plain Text/Source Code File Header)			*
  * @format.use-tabs true	(see http://www.synchro.net/ptsc_hdr.html)		*
  *																			*
- * Copyright 2010 Rob Swindell - http://www.synchro.net/copyright.html		*
+ * Copyright Rob Swindell - http://www.synchro.net/copyright.html			*
  *																			*
  * This library is free software; you can redistribute it and/or			*
  * modify it under the terms of the GNU Lesser General Public License		*
@@ -123,6 +123,7 @@ xpTimeZone_t DLLCALL xpTimeZone_local(void)
 #endif
 }
 
+/* TODO: Supports local timezone and UTC only, currently */
 time_t DLLCALL xpDateTime_to_time(xpDateTime_t xpDateTime)
 {
 	struct tm tm;
@@ -140,7 +141,11 @@ time_t DLLCALL xpDateTime_to_time(xpDateTime_t xpDateTime)
 	tm.tm_min	= xpDateTime.time.minute;
 	tm.tm_sec	= (int)xpDateTime.time.second;
 
-	return sane_mktime(&tm);
+	if(xpDateTime.zone == xpTimeZone_UTC)
+		return sane_timegm(&tm);
+	if(xpDateTime.zone == xpTimeZone_LOCAL || xpDateTime.zone == xpTimeZone_local())
+		return sane_mktime(&tm);
+	return INVALID_TIME;
 }
 
 xpDateTime_t DLLCALL time_to_xpDateTime(time_t ti, xpTimeZone_t zone)