from datetime import datetime


def millis(start_time):
    """
    :param start_time: datetime.datetime 
    :return: milliseconds elapsed from that datetime.
    """
    dt = datetime.now() - start_time
    ms = (dt.days * 24 * 60 * 60 + dt.seconds) * 1000 + dt.microseconds / 1000.0
    return ms