I say ok to use. When I write letters of recommendation, I don't think of there being an expiration date on them. It's someone attesting that, based on their experience with you, they recommend you. That shouldn't change. My only hesitation would be if there was something substantial between when it was written and now that might cause the author to reconsider. That's just going to have to be a judgement call on your part. Editing things gets a little murky...a date? Probably ok. But if you're still in touch with these folks, maybe just shoot them a note and ask if they'd mind you post-dating their letter. That way you don't have to sweat it.