Convert a formula to UDF Function

[jonsonbero]

for illustration the UDF Function in the cell k2
Date of the total amount 1/6/2020
Beginning of the first installment 1/7/2020
The end of the last installment 1/4/2021
With the beginning of 1/5/2021 the cell output will become blank.
What I mean is When I change the computer’s date to 1/7/2020, Then I open the file. The results do not automatically appear.
In this case, I press double-click on the cell k2 to show the results ... This is what happens with me.
How can this be achieved automatically way?.. maybe I'm missing something obvious, so any help at all would be massively appreciated ..