Returns the offset (in minutes) from GMT of the given date and timezone.
offsetFromGMT( date, timezone )
date: (Date) The date for which the offset should be returned.
timezone: (Text) The string Id of the timezone for which the offset should be returned.
You can copy and paste these examples into the Expression Rule Designer to see how this works.
offsetFromGMT(date(2005,12,13), "America/Los_Angeles") returns