def datetime_to_str(dt='now', no_fractions=False):
    """The Last-Modified data in ISO8601 syntax, Z notation.

    The lastmod is stored as unix timestamp which is already
    in UTC. At preesent this code will return 6 decimal digits
    if any fraction of a second is given. It would perhaps be
    better to return only the number of decimal digits necessary,
    up to a resultion of 1 microsecond.

    Special cases:
    - Returns datetime str for now if no parameter given.
    - Returns None if None is supplied.
    """
    if (dt is None):
        return None
    elif (dt == 'now'):
        dt = time.time()
    if (no_fractions):
        dt = int(dt)
    else:
        dt += 0.0000001  # improve rounding to microseconds
    return datetime.utcfromtimestamp(dt).isoformat() + 'Z'