If you're a Windows Phone user, you should have received an update to the stock Calendar app last night. And, if you did receive it, and if you have attempted to create a new item on your schedule, you may have noticed that the app simply force-closes when you try.
Fortunately, or unfortunately, depending on how you look at it, it's not just you. Microsoft delivered a buggy update yesterday that is affecting all Windows Phone users. Whenever you attempt to create a new Calendar item, the app will just shut down. No errors and no explanation.
Microsoft updated the Calendar app to prepare for the rollout of Windows Phone 8.1 to qualifying handsets. What's not clear yet is if the Calendar app update was actually intended for Windows Phone 8 and should not have been delivered to those running the Windows Phone 8.1 Developer's Preview.
I expect a new update shortly. The functions of the Calendar app are pretty important to smartphone users. With Microsoft hinging a lot of success for Windows Phone on the upcoming upgrade, this doesn't bode well for market perception.
Reports suggest that you can bypass the manual creation of a Calendar item, and just have Cortana do it for you. I have tested and verified that this works.