I would guess that you will need to convert the date to a character field in the formula. I assume the character field is not also a date? (But even if it is a date field cannot contain 2 dates!)
In this case I think you will need the DtoC function (Date to Character)
should do it.
Or some variation on that idea to change the order or insert spaces or other fixed characters for example.
Edit: If you look at the included help there seems to be a small error in the example which refers to CtoD rather than DtoC.
CtoD is the opposite conversion and would convert a character format date into a real date field.