Fixed a bug that prevented monthly tags from working in Internet Explorer 11

【Contents of the fix】
■Fixed a bug that the tag for narrowing down the date (year and month) of an event on the top page, event list page, etc. does not work in IE11.

The following files have been modified for this update.
【Fixed files】
・assets/js/event.js
・assets/js/event.min.js
・style.css