Adding "leap days"
People who travel around the globe a lot from west to east may want to be able to add leap days when they cross the Date Line (or subtract them, if going east to west). I've had this problem several times now.
People who experiment with non-24-hour-day eating cycles may want to add or subtract days from a calendar month.
Obscure needs, I know. But it would be cool to be able to add or subtract extra days from a calendar month. There are other solutions, of course, such as ignoring the need to have one's current day be the calendar day. But that seems like a pain.