The function checks whether the given text is a valid time zone.

The function checks whether the given text is a valid time zone name or a valid definition of UTC offset.

Valid names are those defined in Time Zone Database. Compendious list is available on Wikipedia.

Valid definition of UTC offset is text using format "(+|-)hh[:mi[:ss]]", where hh defines number of hours, mi defines number of minutes, and ss defines number of seconds. The sign as well as the number of hours are essential parts of offset definition, number of minutes and seconds are optional and default to 0 (e.g. "+02:30" defines offset of 2 hours and 30 minutes from UTC).