I had the exact same problem with my departing date which was today 30th Fri 30 may( actually on flight as I text) and my arrival 31may.I must have put in a half a dozen time and just had wouldn't accept my arrived date as 31.So I just put the 30 th which was a lie.I then though I better go back and just fill it out once again just incase the Thai custom fail me.Suprise! Suprise! It then accepted the proper dates.Depart 30th today.Arrive 31.