My 2000 SE recently starting pinging really bad under load, so I checked the timing and it was at 20 BTDC. So I reset it to 15 BTDC. How did the timing get off by so much? Does this mean that the timing belt is worn and I need to replace it?

I've had it since October '99 and had no pinging until about 2 months ago. I've always run 87 octane. Yes, it's hot here, but I had no problems duing previous summers.

by the way, it's got 82000 miles

The Timing Belt doesn't stretch (at least not enough to be a problem) but the teeth can wear and break off but if they did the valves would start hitting the pistons. 5 degrees is alot but not enough for it to be damage. Mine changed about 3 degrees over the course of a year in a half so it may just be that it hasn't been checked often enough.

The belt may not stretch.. but the tensioner can lose its spring and put less pressure on the belt causing loosness

It's have to loose alot of tension for the belt to start skipping teeth.