import time


def datetime_to_timestamp(datetime_obj):
    """Convert utc datetime object into milliseconds timestamp.

    :param datetime_obj:
    :type datetime_obj: datetime.datetime
    :returns: Number of milliseconds since epoch
    :rtype:int:
    """
    time_tuple = datetime_obj.timetuple()
    return int(time.mktime(time_tuple) * 1e3)