Convert a wmi formatted timestamp into an epoch.

def _wmi_to_ts(self, wmi_ts):
        ''' Convert a wmi formatted timestamp into an epoch.
        '''
        year, month, day, hour, minute, second, microsecond, tz = to_time(wmi_ts)
        tz_delta = timedelta(minutes=int(tz))
        if '+' in wmi_ts:
            tz_delta = -tz_delta

        dt = (
            datetime(year=year, month=month, day=day, hour=hour, minute=minute, second=second, microsecond=microsecond)
            + tz_delta
        )
        return int(calendar.timegm(dt.timetuple()))