Hi there! I understand you're having trouble changing booking dates in the Park app on your Mac with OS X 10.15.7. Let's see if we can get that sorted out.

First, just to clarify—are you trying to change the dates of an existing booking within the Park app, but the option to modify the dates isn't working or is greyed out? Or is the app not allowing you to select new dates when making a booking?

Also, is the app updated to its latest version? Sometimes older versions can have bugs that affect functionality.

While I wait for your confirmation, here are a few basic steps to try:

1. Restart the App: Close the Park app completely and reopen it; this can refresh its functionality.

2. Check for Updates: On Mac, visit the App Store and verify if there's an update available for Park.

3. Verify Internet Connection: Ensure your Mac is connected to the internet properly since date changes require server communication.

4. Logout and Log Back In: Sometimes, session issues can prevent changes; try logging out and back in.

If these don't help or you've already tried them, let me know, and we can explore deeper settings or account-related issues.

How does this sound?