def rationalize_until(until=None):
    """
    Rationalizes the `until` argument used by other functions. This function
    accepts datetime and timedelta instances as well as integers representing
    epoch values.
    """
    if until is None:
        until = "+inf"
    elif isinstance(until, datetime):
        until = to_unix(until)
    elif isinstance(until, timedelta):
        until = to_unix((datetime.utcnow() + until))
    return until