We have seen this on numerous occasions, you can go to your local immigration office and very politely point the mistake out to them and ask if they can change the date for you and give you your allocated days, if they will not you can go back to the airport and ask the Immigration to please change your stamp to the correct date.
I have come to Thailand with a one year non-immigrant o type visa multiple entry I have just checked my visa stamp that I was given at the airport and they have only stamped me 30 days and not 90 days what can I do?
