Fix iso 8601 time handling

This must not depend on local timezone in any way, so use timegm
instead of mktime and take parsed timezone into account so both
e.g.  2013-08-02T02:09:39Z and 2013-08-02T06:09:39+0400 are correctly
parsed as 1375409379.

Author: AMDmi3

src/osm.c

@@ -93,17 +93,13 @@ static
 time_t convert_iso8601(const char *str) {
   if(!str) return 0;
 
-  tzset();
-
   struct tm ctime;
   memset(&ctime, 0, sizeof(struct tm));
   strptime(str, "%FT%T%z", &ctime);
- 
-#ifdef __FreeBSD__ // XXX: TEMPORARY HACK TO MAKE IT COMPILE ON MY DESKTOP // AMDmi3
-  return mktime(&ctime);
-#else
-  return mktime(&ctime) - timezone;
-#endif
+
+  long gmtoff = ctime.tm_gmtoff;
+
+  return timegm(&ctime) - gmtoff;
 }
 
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