How do I calculate the 90-day reporting period for immigration in Thailand?

This is a guide for reporting the 90-day stay to Thai immigration for expats. It's the responsibility of the individual to track their 90-day period starting from the day they entered Thailand, as immigration does not notify you. Although there may be an unreliable automated email reminder after previous reports, it does not account for your travel history. For the report, you have a 15-day window before and a 7-day window after the due date, which is calculated as 90 days from your entry date. It is advised to visit immigration during mid-week for potentially shorter wait times and to set reminders for future reports.