@classmethod
def utc(class_):
    """Returns a time-zone representing UTC.

        Same as TimeZoneInfo('GMT Standard Time', True) but caches the result
        for performance.

        >>> isinstance(TimeZoneInfo.utc(), TimeZoneInfo)
        True
        """
    if not '_tzutc' in class_.__dict__:
        setattr(class_, '_tzutc', class_('GMT Standard Time', True))
    return class_._tzutc
