I'm currently playing pokemon soul silver and I just discovered that you can actually rematch gym leaders. But the problem is the time and date in my nintendo ds isn't currently set to my current time. So what I'm planning to do is to change its time and date until I get all the contact numbers of the gym leaders so I could re-battle them. Is this possible? Won't it mess up my save data? Thanks in advance!
Changing the clock on the DS will change the clock in-game, and it won't mess up your save. However, your plan still won't work as well as you expect. Whenever you change the time on your DS, you can't do any time-based events for 24 hours after you start your game again. Thus, feel free to change the time to better fit your schedule, but don't expect anything until a day after the change.