IPhone :: Can't Sync IP Calendar To MacBook Pro

I recently purchased a MacBook Pro (15" 2.5gHz) off a friend and am using it as my primary computer. When using iTunes, I click to sync my calendars with iCal and tell it to sync (along with syncing contacts, notes, apps, etc).Everything syncs perfectly EXCEPT for my calendars, which wipe the contents of my phone and replaces it with whatever is on my MacBook Pro. If there is one event or ten events on the laptop, they transfer onto the iPhone and none of my iPhone events transfer onto the laptop and are lost, so I am left to restore from a back up and try again.

I have tried using both iCal and Entourage. I had this problem once with Outlook on Windows 7 in which I lost all my calendar events but the next sync showed no problems. I have tried this sync on my Mac at least 10 times, trying to choose something different each time, but with no luck.

I have looked into this extensively but cannot find anyone with the same problem as me, let alone a solution. I am using Snow Leopard 10.6.4