A family of Microsoft spreadsheet software with tools for analyzing, charting, and communicating data.
If you select a locale in the dropdown beneath the list of available date codes in the date section and subsequently choose a date format in that locale, then the custom number format shows codes like the one you posted.
A list of those codes can be found here:
http://office.microsoft.com/en-us/excel-help/creating-international-number-formats-HA001034635.aspx