This has happened to me a couple of times. Don't know why, but this has worked for me.
Calendar app won't sync events older than 1 month
"OK I finally figured this out. It is a stupid hack but it worked for me and so far my
data is still intact.
1. From home screen hit menu and go to settings.
2. In settings go to Accounts and Sync
3. Turn off the background update and autosync.
4. Go back to main settings menu and go to Applications.
5. Go into manage applications.
6. Press menu button again and select Filter and then All.
7. Find Calendar and select it and delete all data (it will warn you about losing all files
just hit OK).
8. Do the same for Calendar Storage.
9. Go back to main Settings menu and scroll down to Date and time.
10. Turn off Automatic and then set date to January 1st 2009 and hit set. (I tried to go
all the way back to 2006 on my first try and it just kept rolling the date back to the
current one)
11. Wait a second and make sure date remains set to the date you selected. Then
select Date again and select your desired date (should be before first entry on
calendar.)
12. For good measure I then shut the phone off and then powered it back up after
about 5 minutes. (Don't know if it was necessary but I wasn't taking any chances this
time.)
13. Go back into Settings menu and select Accounts and sync.
14. Select the Calendar to sync and then go into calendar to ensure its all there.
15. Once it is you can then safely turn on auto time and date again and let all apps auto update in background. "
I didn't wait the 5 minutes in step 12 and the calendar synced just fine.