What functionality is missing? What is unsatisfying with the current situation?

I am looking to add negative leap days, ie a month is cut short instead of lengthened, to my calendar. Currently only positive leap days are possible. This goes for both calendars and chronicals.  

How does this feature request address the current situation?

If we can cut short the month on a cycle then that would solve my issue.  

What are other uses for this feature request?

I would use this feature to account for the alignment of the new year to the new moon, as I have a intercalary day every season change this offsets things rather regularly. The new year cuts the last month short to start on the new moon. I will admit I’m probably the only person who will use this feature but I don’t see why we can’t have negative values for leap years.