Changes time's timezone (for example to convert from GMT to another time zone).
If time's current timezone is unset (i.e. equal to GDA_TIMEZONE_INVALID), then this function simply sets
time's timezone attribute; Otherwise, it adds or removes hours, minutes or seconds to reflect the time in the new timezone.
Changes time's timezone (for example to convert from GMT to another time zone). If time's current timezone is unset (i.e. equal to GDA_TIMEZONE_INVALID), then this function simply sets time's timezone attribute; Otherwise, it adds or removes hours, minutes or seconds to reflect the time in the new timezone.
Note: the resulting will always be a valid time.