Assuming my actual 90 days Reporting is due on 26th August.

That is exactly 90 days of stay in Thailand. Calculated according

to days and not in months.

I come to understand that I could report 14 days earlier.

If that is true, then could I report on the 15th August ?

I want to report as early as possible, as I have to attend to some

urgent matters.

Therefore is 15th August the right time or any other date to

comply with the 14 days in advance.

Your kind advice is much appreciated and thanking you all in advance.

15 days before, 7 days after.

I am sure that they will not hassle you 16, 17, 18 days early, but who knows TIT.

I suspect the will not allow - the rule is within 15 days before until 7 days after.

Check with your office. almost all immigration offices allow you to apply by mail.

I believe if you apply by mail you don't get the "7 days after" option. Chaeng Wattana says "post 15 days before".

In the past I've also reported by post about a month before (by mistake). They held on to the paperwork and then issued the receipt as normal. No guarantees they'll do this for everyone, though.

The at least 15 days before mail in and no mail in for reporting after the due date really depends on the local office, that is why I said inquire at the local office.

Some will only allow mail-in if you do it at least 15 days before the due date, others wont accept mail-in after the due date and other offices don't mind if you mail it in within 7 days after the due date.

Although there are set rules as to reporting early each immigration office also has their own. I was told by my local immigration office up to 7 days before but not after.

The poster has a report date of Monday, 26th August.

He wants to report on Thursday, August 15th.

No question that this is OK according to the rules.

His earliest date would be Monday, August 12th.