from time import mktime


def toMillis(date):
    """Converts a Date object to its millisecond value elapsed since
    January 1, 1970, 00:00:00 UTC (GMT).

    Args:
        date (Date): The date object to convert.

    Returns:
        int: 8-byte integer representing the number of millisecond
            elapsed since January 1, 1970, 00:00:00 UTC (GMT).
    """

    millis = mktime(date.timetuple()) * 1000 + date.microsecond // 1000
    return int(millis)