Decode a BSON datetime to python datetime.datetime.

def _get_date(data, position, dummy, opts):
    """Decode a BSON datetime to python datetime.datetime."""
    end = position + 8
    millis = _UNPACK_LONG(data[position:end])[0]
    diff = ((millis % 1000) + 1000) % 1000
    seconds = (millis - diff) / 1000
    micros = diff * 1000
    if opts.tz_aware:
        return EPOCH_AWARE + datetime.timedelta(
            seconds=seconds, microseconds=micros), end
    else:
        return EPOCH_NAIVE + datetime.timedelta(
            seconds=seconds, microseconds=micros), end