Outlook Calendar has started to comingle my 2012 schedule with my 2013 schedule.

Outlook Calendar started showing the appointments I had scheduled for 2012 with my appointments for 2013.  When I go back and look at my 2012 calendar my 2013 appointments are showing up as well.  The calendar was fine a few days ago, I turned off my computer Sunday night and Monday morning the calendar is doing this crazy thing.  What is going on and can you please fix this problem?