Excel treats the year 1900 as a leap year and includes Feb 29, which is incorrect. The year 1900 was not a leap year, and so this day should not have been counted.
Surfer starts at 2.0 so that all the numeric values March 1, 1900 and after match Excel’s numeric value. However, that does mean that the numeric values from January 1 through February 29, 1900 are off by 1 between Surfer and Excel. However, Microsoft acknowledges the dates before March 1, 1900 are incorrect in Excel.
Why Excel did this was likely to ensure compatibility with Lotus 1-2-3, which had this bug. They purposely put Jan 1, 1900 as a Sunday instead of Monday. There’s a Microsoft support article about this at: http://support.microsoft.com/kb/214058
Updated October 20. 2017