This is just a simple pop-up calendar you can use to do basic calculations. It can be opened from the Tool Bar at the top of the screen or by double-click the Quantity field while working on an Estimate, Change Order or Estimate Template.
If opened from an Estimate, Change Order or Estimate Template, the value on the calculator will be entered into the quantity field when you click the Close button.
You can either use the mouse to click the buttons on the calculator or use the number pad on your keyboard.