You will have an "enter before date" on your visa. That will be the late date you can enter to get a further 1 year permission to stay. When you return simply write the visa number on the arrival card.
I think it is another of those things that varies from office to office. Buriram Immigration want the letter dated the same day. I believe Surin do too. Just for information. I see that the OP is going to IO in Phuket.
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