code: def ValidateTimezone(timezone, column_name=None, problems=None):
    
    if IsEmpty(timezone) or IsValidTimezone(timezone):
      return True
    else:
      if problems:
        # if we get here pytz has already been imported successfully in
        # IsValidTimezone(). So a try-except block is not needed here.
        import pytz
        problems.InvalidValue(
            column_name, timezone,
            '"%s" is not a common timezone name according to pytz version %s' %
            (timezone, pytz.VERSION))
      return False
docs: Validates a non-required timezone string value using IsValidTimezone():
      - if invalid adds InvalidValue error (if problems accumulator is provided)
      - an empty timezone string is regarded as valid! Otherwise we might end up
        with many duplicate errors because of the required field checks.