We're experiencing a similar issue (Calendar.intToDate is not a function), which is also described here. I'm adding to this topic as the other one is a lot older.
In our case, it's with Seblod 3.22.1 and Joomla 3.10.11, and there's also a UX issue: When the user clicks the calendar icon, the calendar picker pops up. User picks a date and the picker closes/disappears (so far so good). User realizes they picked the wrong date, and click on the calendar icon again ... the picker does NOT pop up this time. Every time the calendar icon is clicked, the abovementioned error shows up in the browser console.