I support some of our users who have P900's or P910i's.
Bit of a tragedy happened the other day in that I was told by a user that after doing some edits on their P900 calendar, they got a message at the bottom of the screen saying 'Tidying' with a progress bar. This went on for some minutes and then completed. It turns out that about 150 days of their calendar (in the past) have been deleted.
They did have around 650 days calendar in total - I haven't come across this before. Is it the OS on the device doing some 'freeing up' of memory or something else? It's worrying to say the least.