If I've read this right, because you are using a formula, it is incrementing on subsequent days when the workbook is opened/manipulated?
The code below will convert your formula based date to a static string date for any selection of date cells (meaning you will need to highlight the range of cells that you would like converted prior to running the code.
1. Make a copy of your workbook and run the following code on your copy (just in case)
2. With excel open, press ALT+F11 to open the Visual Basic Editor (VBE)
3. From the insert menu, select "Module".
4. Paste the code from below into the empty white space.
5. Close the VBE.
6. Select the range of dates you would like converted.
7. From the developer tab, choose "Macros", select "StringIt", and click "Run".