I obtained a single re-entry permit from Nathon Immigration which is valid from 14th May 2015 to 28th December 2015. This latter date is also the date when my retirement visa extension ends. I actually left for overseas on 21st May and re-entered Thailand on 23rd July 2015. I am not clear when I must get my 90 day stamp from Immigration. Can anyone advise?

I assume you are asking about doing your 90 day report.

Your report is due 90 days from the date you entered the country including that date in the count.

The 90 day reporting clock re-starts whenever you enter the country, so it's 90 days from 23 July.

I'll let you do the maths

I obtained a single re-entry permit from Nathon Immigration which is valid from 14th May 2015 to 28th December 2015. This latter date is also the date when my retirement visa extension ends. I actually left for overseas on 21st May and re-entered Thailand on 23rd July 2015. I am not clear when I must get my 90 day stamp from Immigration. Can anyone advise?

Re-entry permits always expire on the same date as your current extension (or in the case of a single re-entry, it would no longer be valid for any more entries once it has been used).

If you do not leave the country again before then, your next 90 days will be on 20th October. You can do the report up to 15 days before that date & up to 7 days after that date.

Edited September 6, 201510 yr by Suradit69

The 90 day reporting clock re-starts whenever you enter the country, so it's 90 days from 23 July.

I'll let you do the maths

Risky

and re-entered Thailand on 23rd July 2015

A common misunderstanding is adding 90 days to the arrival date, while correctly is arrival date + 89.

So the nominal due date is Tuesday, October 20.

As usual the grace period is -14 days to +6 days (officially described as "15 days before, 7 days after").

So for report in person:

earliest date is Tuesday, October 6

latest date is Monday, October 26

http://www.timeanddate.com/date/dateadd.html

Edited September 6, 201510 yr by KhunBENQ

A small clarification to the OP: When you are doing a 90 Day Notification of Residence, you don't receive a stamp in your passport. You receive a slip of paper which you should staple into the passport, if they don't do that for you. I think most offices are now stamping your next due date onto the paper before they give it to you.