When filling out the TM47 form, the 'Date of Thailand Arrival' refers to the last arrival date stamp you received upon entering Thailand. Several comments confirm that this is always the latest date you entered the country, making it the date of your most recent entry.
Bart **************
Last arrival.
Mike *********
That’s odd as I have always used my initial entry into Thailand and they get approved all but 1 time and that denial was because of the system update.