When should I complete the TDAC form before arriving in Thailand?

Can you count up to 72 hrs before arrival? That would be it.

maximum 72 hours before ARRIVAL

What do you mean it's unclear?

After

It means after 7th Sept. Anyway system will not let you fill earlier

You can do it when you land also. I normally do it after I’ve checked in and killing time at the airport before travelling to Thailand 🇹🇭 🙏🏽👌

Either the night/day before your departure after done with your packing, or in the departure lounge at airport if you have time. If they have onboard wifi with your flight then that could be an opportunity to do it. If you have a sizeable layover somewhere then another good opportunity. If you have family / kids etc then probably before all mad rush packing when you are still at home but within 72 hours of your arrival time. Although not advisable the last opportunity you will get is after you step off the plane and connect to the website through wifi before going through immigration.

I did mines 7 hours before my flight. Some people did it on the plane. There's literally a kiosk (at least at DMK) to complete it there.

No urgency needed. If you wanted to really risk it, you could probably do it in the immigration line.

To answer your question. If you're set to arrive on let's say the 5th, it won't allow you to fill it out on the 1st. Needs to be 72 hours within the time of your arrival.

Up to 72 hours before arrival. After 7th.