I had the same problem and my work is very dependent on my calendar. After speaking with a senior advisor over a period of two weeks, I decided to download my calendar from 2001-2015 to yahoo and google. Then I had Apple delete all event prior to 2015. Now on my icloud I only have events from 2015. While I have tried to open the yahoo or google calendar into icloud i have not been successful. If anyone has suggestions I would be happy to hear!
Thanks