Calculate 180 days from your arrival so you can check the exit date when they write it in your passport.
Yesterday the passport lady had to recalculate and cross out her original date and (hopefully) put in the correct one.
TLDR : Answer Summary
To determine your 180-day stay in Thailand, start counting from your arrival date as Day 1. Be sure to verify the exit date written in your passport, as it may require recalculation if there is an error.