[formatter] fix local DST datetime offsets for ':O'

'O' would get the *current* local UTC offset and apply it to all
'datetime' objects it gets applied to.
This would result in a wrong offset if the current offset includes
DST and the target 'datetime' does not or vice-versa.

'O' now determines the correct local UTC offset while respecting DST for
each individual 'datetime'.

gallery_dl/formatter.py

@@ -375,18 +375,18 @@ def _parse_offset(format_spec, default):
     fmt = _build_format_func(format_spec, default)
 
     if not offset or offset == "local":
-        is_dst = time.daylight and time.localtime().tm_isdst > 0
-        offset = -(time.altzone if is_dst else time.timezone)
+        def off(dt):
+            local = time.localtime(util.datetime_to_timestamp(dt))
+            return fmt(dt + datetime.timedelta(0, local.tm_gmtoff))
     else:
         hours, _, minutes = offset.partition(":")
         offset = 3600 * int(hours)
         if minutes:
             offset += 60 * (int(minutes) if offset > 0 else -int(minutes))
+        offset = datetime.timedelta(0, offset)
 
-    offset = datetime.timedelta(seconds=offset)
-
-    def off(obj):
-        return fmt(obj + offset)
+        def off(obj):
+            return fmt(obj + offset)
     return off