Sorry if this has already been answered before but I spent a moderate amount of time searching and couldn't find it.

I arrived back in Thailand on October 14, 2014 and have remained here since then.

I renewed my (second) extension of stay based on retirement on December 24, 2014.

When must I do my next 90 day reporting, 90 days from Oct. 14, or 90 days from Dec. 24?

You need do a report 90 days from when you entered the country on October 14th.

Only the first extension of stay counts as a 90 day report.

They always put a slip of paper with my next 90 day reporting date into my passport after I get my extension,

Then another slip with the date of the next one after the 90 day.

I always check the date on the slip before leaving the office..sometimes they calculate the 90 days wrong

I would not leave the office without the slip with the next report date on it!

You need do a report 90 days from when you entered the country on October 14th.

Only the first extension of stay counts as a 90 day report.

Thank you ubonjoe for confirming this. I had a feeling this would be the answer as stupid as it seems to me to have to do a 90 day report to state my address, only 3 weeks after providing my address to immigration while getting my extension of stay. Not the only stupid rule they have here but we must comply or face the consequences ;-)

They always put a slip of paper with my next 90 day reporting date into my passport after I get my extension,

Then another slip with the date of the next one after the 90 day.

I always check the date on the slip before leaving the office..sometimes they calculate the 90 days wrong

I would not leave the office without the slip with the next report date on it!

Unfortunately they didn't give me such a reminder slip. I wish they would have because it would have been clear if they did.