My iPod touch clock setting is always changed by itself! Help!!?

I bought this 2nd gen iPod touch in the middle of '09. Used it in Singapore timezone (GMT +8) for about a year. Then I moved to Jakarta which has GMT+7 timezone. I changed the timezone on it. But I don't know why and how, it always changes it-self's clock back without any of my interference on settings. Only the clock, not the timezone. Whenever I change it back, and open some apps (both original and purchased apps), then it just….changes one hour earlier. Again and again. I thought it caused by some events or appointments I made on Calendar apps, but then I remember that I had deleted all of the appointments before I moved to GMT+7. I have tried to restore it on iTunes, but still…..