Hi,
First of all I guess the reason that you see the “Mmm-yy” option in the list is that you have entered that format manually once before. I didn’t have this but since I entered the format, it appears in the list for me as well.
To solve your issue:
Use a string manipulation and join function like this: join(“01-”, $Date$)
Now in the string to data&time node use this format: dd-MMM-yy (the “New type” should be Date, not Date&time and use 3 capital M for month in Date format settings)
And problem is solved.
I also found this topic similar to your issue. Maybe it’s a good idea to take a look:
Best,
Armin