Calendar entries disappeared

[Dansole]

I've had this happened to me at least two times since 2012 - for whatever reason my stock Calendar has dropped all of my calendar entries starting about a year back - the entries still exist on Google Calendar - the last two times I had to go back to Google calendar and add a space in each of the entries and then they would repopulate. Annoying to say the least - Any ideas why this is happening and how to prevent?

Thanks