Fix 1904 dates

We don't need the 1900 leap year bug adjustment code (the + 1 or - 1) when calculating 1904-based Excel dates. Plus the 1904 dates are based off January 1 1904, not December 31 1903. The previous code was causing 1904-based dates to be off by two days.
 
For an explanation of Excel date representations, see http://support.microsoft.com/kb/214330, http://answers.oreilly.com/topic/1694-how-excel-stores-date-and-time-values,  http://www.cpearson.com/excel/datetime.htm