def combine(cls, datepart, timepart, tz=None):
    """Combine date and time into timestamp, timezone-aware.

    Args:
      datepart: datetime.date
      timepart: datetime.time
      tz: timezone or None
    Returns:
      timestamp object
    """
    result = super(BaseTimestamp, cls).combine(datepart, timepart)
    if tz:
      result = tz.localize(result)
    return result