I think I have it. The first day in the 10,000 day calendar is July 29, 1961. If you use an Excel spreadsheet function 'datevalue' it returns the serial number of July 28, 1961 (the day before) as 22490. With July 29, 1961 being 0001, if you add this to 22490 and use the Excel 'month', 'day', and 'year' functions, you get 7, 29, 1961 respectively. I tried this with some dates from the 1970 parts book and if you add the 10,000 day date to 22490 and apply the Excel functions, you get agreement every time.