Steps to reproduce:

1. Create a date macro with "//". Note that the date template is: dd-mm-yyyy
2. Observe that the value of Date Format (on a vanilla instance) in Confluence Admin > General Configuration is set to MMM dd, yyyy, which would be mm-dd-yyyy in the date macro's format.
3. Change the value of the Date Format to yyyy-MM-dd
4. Observe that the date macro's template remains unchanged