Re: Z10 need to periodically reset
Calendar entries are NORMALLY on the cell phone's internal memory not the memory card.
Try pulling the memory card and only using internal memory for a few days or how ever long it takes to see if it helps. If you using a third party app for calendar use try the base calendar on the Z10.
I have two Z10's and never had any issues with calendar except when my computer over wrote the entries on the phone and added old data. That was caused by me not selecting the proper sync options.
Also do not let the phone battery power go so low that it may cause issues. It seems like your changing battery's and the issue is resolved.
Z10 on BES
Z10 on BIS