Just a note of experience, I went to Chaengwattana January 2nd to cancel my extension. The letter from my school said my last day was December 31st 2019. They charged me 2 days overstay ! Even though they were closed on New Years Day January 1st. They start counting from your last day of work.
TLDR : Answer Summary
A user shared their experience of being charged for an overstay when they went to Chaengwattana to cancel their visa extension after their last working day. Despite the immigration office being closed on New Year’s Day, they were charged for two days of overstay because the counting began from their last day at work. They subsequently applied for a 7-day extension, which lasted until January 7th, before completing a visa run to Myanmar for an additional 30-day visa.