In (rights) permissionStatement.termOfGrant the start and end date of
the permission granted is required and ISO 8601 format is recommended.
But what if something has no real start (e.g. fair use) or no end
Seems like you wouldn't want to say "none" because that wouldn't pass
parsing. Perhaps you could use "0000" for no start date and "9999" for
no end date -- this seems to be what is done in the PREMIS examples.
Is there a standard convention that people use?