I am working on an application where the horizontal axis of the Visio page represents dates from left to right. In fact, the each of the fixed horizontal grids are 1 day, and I need to have shapes that understand the begin date at the left edge of the shape, and the end date at the right edge of the shape. There is, therefore, a number of elapsed days representing by the width of the shape. However, the underlying grid can either represent all days, or it can be changed to only represent weekdays by omitting the weekends. I struggled to find the best formula to calculate the elapsed days or weekdays between two dates in Visio ShapeSheet formulas, so I turned to Excel to provide an inspiration. The Visio ShapeSheet is modelled on the Excel worksheet, and formulas can be entered into the cells in much the same way. However, the available functions differ since Excel is mainly used for arithmetic and statistics, but Visio is used for graphics and data. In this article, I demonstrate how I used C# and Excel to construct and test formulas for use in a Visio shape.