That's an interesting one. The DS1307 is completely naive of timezones so the issue would be with the time your computer is putting out somehow not compensating for DST. I presume you set the clock using the timerset.pde sketch?
Oh- I forgot about that separate sketch. My DS1307 must still be running from the last time that I set it. That says a lot since it's been out in a drawer collecting dust for the last six months,LOL. I guess the back up battery must have kept it alive.
I'll have to try that sketch. I just adjusted the start and stop time in the main sketch to compensate for now. hehe!