Handle changes to TZ (e.g. whether DST exists)
The DST (and possibly other) information is extraced by assuming that the current year stays the same. However, this assumption may not hold true, and DST information may change between years.
As such, both gweather_timezone_has_dst
and gweather_timezone_get_dst_offset
probably need to check whether the original assumption still holds before returning the value. One should probably double check parse_tzdata
to make sure to get the condition exactly right.