hmmm..... check your dates indeed. In a normal situation you would expect to be able to rely on the dates stamped in your passport. However such is the arrogance of Thai immigration they deem incorrect date stamps to not prevail and they will fine you for any overstay. This has happened to me and brings up issues of staff incompetence ( probably not an issue in Thailand?) and money-making at every step in their immigration processes whether rules are followed or not. Remember Mr/Mrs/Miss Tourist a.k.a "the alien" , its not you the Thais want , but your money