@Kathy ***************
I believe you are confusing TWO completely different things; the visa validity with the number of days that visa gets you stamped in to the country..
A single entry tourist VISA is valid for 90 days from the date it's issued. <- That means you have 90 days to ENTER thailand.
Even if you enter the country on the last day the visa is valid for you will still get stamped in for 60 days like everyone does who enters on a tourist visa.
AND
you can apply for one 30 day extension on that 60 day entry stamp from a tourist visa at the immigration office for 1900baht
You're confused is all.