I did open the tbwx file and wondered few things. Why are you focusing to change the null values for the last 24 months only? I guess it would be straightforward to change the whole sales null data to zero with ZN formula.
I would also suggest to use relative date filter inbuilt inside Tableau desktop for dynamic last 24 months case.
Could you elaborae what do you mean " I was able to replace Null with zero for dates in between but not for dates at the beginning"?