I was pleasantly surprised to discover that the calculator can quickly and painlessly calculate the differences between two dates, or add or subtract days to a specific date. To access this functionality, just fire up Calculator and go to View > Date Calculation . You can also hit Ctrl-E to get there with the keyboard

38a34f1a92r 9606.png 350x187 Use the built in Windows calculator for date calculations

The rest is here:
Windows tip: Use the built-in calculator for date calculations