Hi. Thanks for your question and the screenshot.
Here’s what I see on my default/testing install with Twenty Fifteen theme:

In other words, there’s no date whatsoever in that field so I’m not certain where it would be coming from.
Also, I saw this in the Chrome Console at your site’s Events page:

I know you don’t want to switch themes, but my suggestion with the information I have is to please follow our Testing for Conflicts Guide and see if that helps narrow down the cause of this. (Hopefully it’s a plugin issue and not a theme issue, or something in your theme’s functions.php file that you can comment out to test.)
Let us know what you find out.
Thanks.