Start with a blank document.
1. Insert>Fields>Other
2. Select Date, Additional Formats...
3. Enter format code D MMMM YYYY
4. If you want, edit comment to My New Date Format, or whatever you like
5. Click OK
6. Click Insert
7. Save document and close
8. Re-open document - date format IS the user format entered above.

Create a new blank document
1. Insert>Fields>Other
2. Select Date.
3. The format entered above shows in the Format list.

Good so far.

Open an existing document created prior to the above containing a date field 
NOT in the above format.

1. Insert>Fields>Other
2. Select Date
3. NOTE: format entered above does not show in the list.
4. Select Additional Formats...
5. Enter format code D MMMM YYYY
6. If you want, edit comment to My New Date Format, or whatever you like
7. Click OK
8. Click Insert
9. Save document and close
10. Re-open document - date format is not the user format entered above.
11. Insert>Fields>Other
12. Format entered above does not show in the list.