def transform_iso_datetime(iso_datetime):
    """
    iso date times are of the form 2018-03-22T11:12:14, this is not a valid
    file system name and oses (osx for example,) transform the ':' into '/'
    which is not what we want in our bucket object names.

    We transfor the ':' into a '-' so that the iso datetime becomes
    2018-03-22T11-12-14.
    """
    return iso_datetime.replace(':', '-')