on playing around a little more, I have discovered that a little bit more is needed to correctly enforce the intervals.

In calendar_stripped.js:

insert
mins=(mins>=cal.params.minutesInterval)?mins-(mins%cal.params.minutesInterval):0;
after
cal.onSetTime=function(){var pm,hrs=this.date.getHours(),mins=this.date.getMinutes();

Insert
cal.onUpdateTime();
after
M.innerHTML=(mins<10)?("0"+mins):mins;

This will make it so that the actual time rounds down to the closest available allowable interval both in the calendar display, and in the actual time that is set. this will happen as soon as the calendar is opened.

One benefit of this is that it fixes an error where the word "undefined" shows up in the minutes area of the pop-up calendar when one tries to change by dragging immediately after opening the calendar.