>>> date_to_datetime(date(2000, 1, 2))
    datetime.datetime(2000, 1, 2, 0, 0)
    >>> date_to_datetime(datetime(2000, 1, 2, 3, 4, 5))
    datetime.datetime(2000, 1, 2, 3, 4, 5)

def date_to_datetime(d):
    """
    >>> date_to_datetime(date(2000, 1, 2))
    datetime.datetime(2000, 1, 2, 0, 0)
    >>> date_to_datetime(datetime(2000, 1, 2, 3, 4, 5))
    datetime.datetime(2000, 1, 2, 3, 4, 5)
    """
    if not isinstance(d, datetime):
        d = datetime.combine(d, datetime.min.time())
    return d