You make your next report 90 days after you re entered. +/- 7 days.

If the due date is counted as 1, isn't the window actually +/- 6 days?

http://www.immigration.go.th/nov2004/en/base.php?page=90days

Procedure and notification:

The foreigner makes the notification in person, or

The foreigner authorises another person to make the notification, or

The foreigner makes the notification by registered mail.

The notification must be made within 7 days before or after the period of 90 days expires.

The first application for extension of stay by the foreigner is equivalent to the notification of staying in the Kingdom over 90 days.