It would be good improvement to support fractional time zones in xml skin files.
Now is not possible to show time in "world time skin" for such time zones as:
India, Mumbai or Bombay GMT+5:30;
Darwin, Australia GMT+9:30;
Chatam Island, New Zealand GMT+12:45;
Kabul, Afghanistan GMT+4:30;
Nepal, Kathmandu GMT+5:45;
Tehran, Iran GMT+3:30 and so on.
Moreover, fractional time zone allow to improve greatly accuracy of dynamic world map of day and night zones (Vitrum skin). This last, in fact, is main reason of my post.

Unfortunately the datetime class in wxWidgets doesn't support those.

I guess that's not quite correct. Quote from http://www.wxwidgets.org/manuals/stable/wx_wxdatetimeoverview.html#tdatetimezones:

To see how to do it, the last issue to address is how to construct a TimeZone object which must be passed to all these methods. First of all, you may construct it manually by specifying the time zone offset in seconds from GMT, but usually you will just use one of the symbolic time zone names and let the conversion constructor do the job.

TimeZone is an inner class to wxDateTime which accepts an offset in its constructor.

That is true. I have used only the predefined timezones so I didn't notice that there is a separate class for them too.

So yes, I can add support for also the fractional timezones to Rainlendar .