Ah, somebody is talking about computer problems. Me is awake!
So good to hear it's still happening, even though a paper calendar isn't really doing it for me at the end of March.
On the other hand: Looking at it not as a calendar but rather an arts object, things are different. So I could simply consider it as a couple of art prints for my EQ collection.
Backups are always a good idea, indeed. And sometimes, once you put a lot of work into something, it is good to put a copy at least of that very file somewhere else.
(I had my share of losses throughout time, dating back to the i486 days. Software error, hardware failures, or even my own stupidity.)
What strikes me is this "happened to me more than once". If it was on the same machine this might be a hint of a systematic error. Multiple reasons are possible, but worth to be checked, so you can decrease the number of events in the future. But I'd need more input on that matter to be of help.