There is a bug with latest Joomla 3.7.4 (and I believe higher versions also).
The thing is: I'm using integrations, in particular, JTicketing. And I need to be able to use standard Joomla field "calendar". But their script conflicts with your old version. As a matter of fact, your calendar script does not work when Joomla "calendar" field is on the page.
It troughs exceptions when you click calendar button. I've tried to avoid those by disabling the script (put empty calendar.min.js at templatefolder/js/system/fields), so I'm able to open the calendar, but updating the valued doesn't work then.
How can I make your field to work?
Version : 3.4.43 Stable