Create a new DateTime corresponding to the same instant in time as
this, but in the time zone
This call can fail in the case that the time goes out of bounds. For example, converting 0001-01-01 00:00:00 UTC to a time zone west of Greenwich will fail (due to the year 0 being out of range).
You should release the return value by calling
g_date_time_unref when you are done with it.
the new TimeZone
a new DateTime, or null