There can be a different due date for your 90-day report if your immigration office gives it to you. This happens when I file by mail 15 days before the due date. Otherwise, the 90-day report is due on the ninetieth day after you enter the country. These dates have no relation to TM 30s or period of stay renewal dates. I have always mailed in my 90-day reports and have never had a problem. Some people don't like doing the paperwork but it always works whereas the online reporting sometimes doesn't work for a variety of reasons. 90-day reports should be mailed not later than fifteen days before the report is due. I will be getting a new passport next year so I will go to report my 90 days in-country in person and have my stamps transferred to my new passport at the same time. After that I'll go back to mail-ins.
Of course, if you are over 5 days late in submitting your 90-day report, you must go in person and pay your 2,000 baht fine.