This is going to be a simple post, but when I ran into this problem a few weeks back I couldn't find any documentation helping me troubleshoot the problem so I am just going to put this short info up here for whomever it might help.

If you are using MEC Calendar, MEC Calendar Pro or any other Event Plug-in that has a countdown feature, you are going to want to make sure the file path is excluded from your caching system.

To exclude the file path simply enter the following paths as an exclusion to your CSS cache.