The native Calendar for Windows Phone 8.1 received an update, but to dismay of those who updated, it brought an issue with itself. It crashed when trying to add an event or calendar entry. Joe Belfiore has just tweeted saying that fix for this calendar crashing bug is ready and is being pushed to the store.
WP8.1 Calendar fix is ready and getting posted to the store. Will take a bit of time to show up everywhere. Sorry for the inconvenience!
— joebelfiore (@joebelfiore) July 2, 2014