def openTypeHeadCreatedFallback(info):
    """
    Fallback to the environment variable SOURCE_DATE_EPOCH if set, otherwise
    now.
    """
    if "SOURCE_DATE_EPOCH" in os.environ:
        t = datetime.utcfromtimestamp(int(os.environ["SOURCE_DATE_EPOCH"]))
        return t.strftime(_date_format)
    else:
        return dateStringForNow()