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.

Updated November, 2021