Not only that, but say you show boarding passes to prove you boarded a plane into Thailand on day A and out of Thailand on day B, how do you prove that you weren't out of Thailand and into Thailand again between day A and day B? Up to now you could tell the tax dept it was impossible to either enter or leave Thailand without the Thais putting a stamp in your passport. Once the tax dept knows the Thais are no longer stamping passports, the only formal proof of your stays in Thailand is through some kind of certificate issued by Thai immigration.