The e.start() and e.end() functions are now returning the new DayPilot.Date object.
It's a wrapper object that removes the time zone information from the date and adds some useful functions.
For documentation, see www.daypilot.org/daypilot-date-object.html.
You can get the underlying Date object using e.start().d (then you will have the same object as before 5.1).