The Calendars.app on my Mac hasn’t worked correctly in a few weeks, the calendaragent process would run wild and sap my battery. Lucky for me, the temporary workaround was to not use Calendars.app on my Mac and my iPhone’s calendar worked fine.

After reading Apple’s Forums (follow the tip from “ST Munees”) and an older Chris Breen Macworld article I was able to fix Calendars.app on my Mac. I can elaborate more later, but following those tips got mine running again and calendaragent is behaving again.