Set or get the current timezone (including dst) rule.
set_timezone returns a new calendar object,
as the called calendar but with another set of rules.
Example:
> Calendar.now();
Result: Fraction(Fri 2 Jun 2000 18:03:22.010300 CET)
> Calendar.set_timezone(Calendar.Timezone.UTC)->now();
Result: Fraction(Fri 2 Jun 2000 16:03:02.323912 UTC)