30 days notice and then leave earlier and want rent prorate back

[Todd Powell]

I have been a landlord since 2008, but some items never come up. Here is the question, a tenant sends me December rent and gets to me on the 5th via first class mail. Along with the rent they have a written notice telling me this is my 30 days notice, but they say they will be fully vacated by Jan 12th. Let me get this straight, on Dec 5th they give me 30 days notice, but state be out by Jan 12th, and then they text me mid December telling me they will vacate by Dec 7th, which turns out to be Dec 5th and they are gone. They never paid for any Jan rent, so I will deduct those days from their security. The question is, does the written letter hold up through Jan 12th, or does the text hold legally on the 7th? I marketed the unit and is rented Jan 15th, but I could not market sooner as their formal letter stated the 12th. Of course tenants can change their mind either way and it seems hard for me to enforce. If they chose to leave after the 12th what would I have done? Curious if I am entitled rent through the 12th, or I am obligated to pay them that week. IT seems small but these tenants have been challenging in other ways. I believe Oregon law states they can give me 30 days notice during any portion of the month, although I had them sign an addendum stating it has to be the 1st of the month for 30 days notice, which of course they did not follow. Thanks for any input you have!