Yes that has happened to me as well, but with a multiple entry non imm O.
He has to to to Immigration and ask them for a form (as it is not out with the "normal" forms that are on display). Fill out the form & have copies of the, I think, back page of passport, Visa stamp & actual Immigration entrance stamp. Immigration will then amend the date on the entrance stamp. No charge - unless you haven't got all the copies they need
I always double check the date given me on entry now, carefully!!
I do caution you to be careful about the 30 day tourist visa extension. I would ask the Immigration officers for "advice" on what it is best for you to do rather than physically apply for one. You could have the forms filled out (in your back pocket) if you got a positive response from him. My experience is that they will only give you a 7 day extension if you ask for an extension. That will cost you 1,900 baht & 7 days later you will have to go to Laos anyway..
