Transform year, month and day to an Excel serial date
static double DateToSerial( int yy, int mm, int dd );
Year (eg 1996).
Day of month (1-31).
An Excel serial date value.
This function calls Excel's DATE() function to calculate a serial date from its components.
Because it uses Excel for the transformation, it handles variations in the Excel base date transparently.
If a call to the Excel API fails, then an exception of type CXlApiException will be thrown. Since this exception is derived from CXlRuntimeException, it will be caught by the add-in wrapper function when this method is called from within an add-in function.