As @Riny_van_Eekelen has already told you, the DATEDIF function isn't really supported by Microsoft except as a carry-over. I went to another website I often find helpful, and it made the same point more colorfully.
Here's a link if you wish to explore further: https://exceljet.net/excel-functions/excel-datedif-function
But your question is intriguing. I can envision a solution that could take advantage of some newer features of Excel (I have the LET function in mind). It would take some elaborate (??) conditional thinking,
- IF the number of days as in your formula is negative,
- then reduce the month by one,
- then find out how may days that month normally has,
- and if it's a February, determine whether it's a Leap Year,
- and then calculate which day it is by subtracting the negative day number from the appropriate .... (you get the idea)
Before I spend some time, which I might do just for the sake of satisfying my intellectual curiosity, let me ask you what version of Excel you are working with.