This article explains how the option 'Excel-style Julian date format' works in Statistica 
 

Using Excel-style Julian date format in Statistica

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The Excel-style Julian date format is the default setting in Statistica (found by selecting File, then Options, expanding Documents node and selecting Spreadsheets) ,and this option affects the way in which dates are interpreted and displayed when a date format is selected as the display format for variables. Statistica deliberately replicates the algorithm used in Microsoft Excel to convert Julian dates due to the fact that the original Microsoft Excel incorrectly recorded 1900 as a leap year, and it has continued this in every version since. When using the Excel-style Julian dates option in Statistica, 0 is undefined and 1 is 1/1/1900. This is why the year 1900 is calculated as a leap year. 

To correctly calculate the number of leap years in Statistica: 

1. Open Statistica
2. Go to File | Options 
3. Expand node for Documents, and then for Spreadsheets
4. Click Navigation Defaults
5. Select the check box next to 'Use Excel-Style Julian Date Format' to deselect this option

This change will correctly calculate the number of leap years in Statistica