from typing import Set


def _accepted_date_formats() -> Set[str]:
    """
    Return all known accepted date formats.

    We expect that more formats than this will be accepted.
    These are the accepted ones we know of at the time of writing.
    """
    known_accepted_formats = {
        '%a, %b %d %H:%M:%S %Y',
        '%a %b %d %H:%M:%S %Y',
        '%a, %d %b %Y %H:%M:%S',
        '%a %d %b %Y %H:%M:%S',
    }

    known_accepted_formats = known_accepted_formats.union(
        set(date_format + ' GMT' for date_format in known_accepted_formats),
    )

    return known_accepted_formats