Hebrew Lunar Calendar
Excel gives calendar support for the Arabic calendar.
I am sure there are many highly technical users working with spreadsheets with arabic calendars (pyrotechnic engineers? Pilot trainees?)
Certainly Microsoft is making enough money off the backs of the many Jewish engineers working for them in Israel, that they could finally get around to converting the Lunar calendar, which would Jewish Excel users to sort a column on Jewish dates. And please add it as patch for Office 2011, as many are more comfortable with that version vs later revs.
PS - Someone did most of the work here, but it still needs to offered as a built-in for Format Cells/Dates.