When applying for 90-day reporting online, should I enter my arrival date in Thailand or the date of my last reporting?

Date you last entered Thailand from a foreign country, even if only a short trip. I.e. the last stamp on your passport entering Thailand (not the 90day date)

First 90day at a new address can't be online

More and more offices are following the "from 2 weeks before until 1 week BEFORE" rule for online applications <- meaning you have a one week window to apply online.

example your 90 day report is due Dec 18th, that means you can apply online from the 4th until the 11th (2 weeks before down until 1 week before)

people have been rejected outside that "window" at more and more offices, Also make sure you match the address EXACTLY with the TM30 you have filed because those two programs now cross check each other against passport/address

AND

remember the first time you go to report after comin' back into the country you have to do it in person (or by mail) before you can use the online 90 day reporting system

On these two fields

The "Arrival Thailand Date" you put the date of you most recently entered thailand (your most recent entry stamp date)

The "Stay Visa Expire" you put the date of your extension expiration (or if you're DTV or OA visa you put the date of your current entry stamp expiration date)