I saw on an article here that April 18th 2025 is the deadline to file a 2021 tax return that was never filed. I thought April 15th was the deadline?

I don't think I can make it to the mailbox in time for the cutoff today to get it postmarked April 15th but if April 18th is in fact the cutoff for filing this 3 year old return that would be wonderful. The recovery rebate credit was never claimed for 2021