Apps are great - they make life so much easier
We all use apps (your employees use them). Sometimes however, they start misbehaving and act in an unpredictable way. Unsurprisingly, the fifth most popular cause of data loss in the cloud is data being overwritten by third party software. If this sounds unlikely then remember it if happened to Steve Wozniak, Apple Co-Founder, then it can probably happen to you!
So what do you do - call Microsoft? Once again, Microsoft will tell you that there is no guarantee that they can recover all (or any) of the files and if they can it may take some time (hours/days/weeks)
With a cloud backup solution you can recover any data that has been overwritten (including calendars)