I too refer back to 'old' calendar entries. I recently experienced a situation where my outlook cal entries older than the last 30 days were gone. (i am certain it was a result of something I did.... ) Anyway, here is how I remedied it.
With your BB connected, open DM, select 'synchronize'(dbl click), select the configuration tab, select 'configure sync', select/highlight 'calendar', change from 'wireless sync' to 'outlook calendar'. Then hit the 'configure' button, go to 'advanced settings', then 'date range'. Under 'options', I selected 300 days prior and 300 days after today(or whatever range you want).
Hit ‘ok’, 'ok', 'close'. Then again select 'synchronize' (dble click), and hit 'synchronize now'.
It SHOULD load everything that is on your Outlook calendar... for the last 300 days.

Then (and finally !!!)...go back and switch your calendar option BACK to wireless sync. And change 'date range' back to 'transfer all scheduled items'.
In my case, i can now go back to late january as a reference point for past appointments.

I futzed around with mine for a little too long before I did the above... this worked for me but there may be an easier way to do it....
Hope it helps.