Date & time settings-iphone automatically changes date & time

So many guys out there are having a problem of date & time on their iphones/ipodes. Why date and time changes automatically which ruins the whole day and we cannot connect to the appstore as well So the only solution is *Not to full your iphone capacity or if you bought iphone only because of gaming so dont install any kind of thing like Dreamboard,winterboard because iphone 3gs has a low processeser