The full Microsoft Office suite.
https://www.microsoft.com/en-us/microsoft-365/microsoft-office
But, to be honest, for your situation, I would leave as is, i.e. status quo. Continue to use your new Calendar for the reminder of 2021, and going forward. Then, use the old corrupt one, for historical purposes.
Clearly, there is an issue in the old Calendar, and it is more than likely related to an item on said Calendar. Exporting and then Importing into a the newer Calendar, might yield the same issue...