import calendar


def datetime_to_epoch_timestamp(a_datetime):
    """Return the epoch timestamp for the given datetime.

    :param datetime a_datetime: The datetime to translate
    :rtype: float

    >>> a_datetime = datetime.datetime(2013, 11, 21, 1, 33, 11, 160611)
    >>> datetime_to_epoch_timestamp(a_datetime)
    1384997591.160611
    """
    return (calendar.timegm(a_datetime.utctimetuple()) + a_datetime.microsecond / 1000000.0)