90 days starts when you enter Thailand, not leave.
You make the report when being in the country 90 consecutive days, period. It's the grace period to report, it's not a period to extend the reporting. It's pretty darn simple. It doesn't mean you can leave without reporting requirement from day 90-97.
The report is due to report that you have spent 90 consecutive days in Thailand. Not 91 or 97. The 7 days after is a reporting grace period to DO the report in person. It must be done.
89 days from the entry date. You have a 3 week window to report, 15 days before or up to 7 days after the due date. In most cases you need to do the first 90 day report in person. You can do online or via mail before, but must report in person if on the due date or after.