I just had another thought.
If you're just using the formula to format your date field, there is a more direct formula-free approach.
You might try the following:
1. Create (Insert) a Text Object in your report.
2. Insert ( Drag & Drop) your Date Field into the Text Object - Twice.
3. Resize the text object to allow for the 2 lines.
4. Use the "Enter"key to place the 2nd Date field under the 1st date field.
5. Right-click on the 1st date field and choose the Format(Datefield) option.
6. Click on the "Customize" button.
7. In the formatting options, select "None" for the Month & Day
8. For the 2nd date field, do the same thing, only choose "Mar" for the Month and "None" for Day & Year.
Now you have options.