The one time I didn’t check it there and then, it had to happen.. The immigration officer in Suvarnabhumi airport forgot to put an arrival stamp on my passport (the one that states arrival date and “admitted until” date). He did put the “used” stamp with date on the visa in passport and also on the white arrival card.

I entered on a single entry tourist visa and want to apply for the 30 day extension in two months. Should I go to my local immigration office right now and explain what happened or can I just wait until it’s time to apply for the extension?

You need to go now and straighten it out. If not and get stopped thay might think that you "dropped in" by parachute!

It would be best to do it now. That way you will not have to worry about having a problem when you go for your extension.

I would try and rectify it now.

If the arrival card doesn't have an entry stamp also, bring the details of your flight you arrived on with you. Boarding pas would be nice, but flight number and date will be enough for them to find the date you entered.

OK, thank you guys for your quick help, I will go to immigration shortly. The stamp in the arrival card is very blurry and the date is hardly visible, looks certainly suspicious even to me.. Luckily, I saved the boarding pass.

Just a thought: Can I apply for the extension at the same time, even though its due in 2 months?

Normally they will not do the extension until you only have 30 days left (two weeks at some offices for tourist visas). But you could try.