Note that this result also needs to be formatted as a number. Then click to select a cell that you want to calculate a future date based on. Excel accepts dates from 1900 to 10000 years.
Copy back to original workbook and see if there is any difference.
Enter 01032016 into column A1. In the example shown the formula is solved like this. A1 100. In the formula A2 is the start date B2 is the days you want to add.