This is a bug I have already encountered last year when adding the ayatana indicators to Mint Mate 21.
I assume you did set a task in Evolution that has a due date, but lacks a starting date.
Whenever I do that, I can reproduce this malfunction.
If you have got such a task in your calendar: Add a starting date to your task or delete the task, or remove the due date of the task. Then log out and then back in, and then the date time indicator will work properly again.
Might be your problem is caused by something else, though. But it is worth checking!