I have found an issue with the subject function. For my testing I have the following:

and I have the following code. 

input.Number_of_Days = (input.Claim_Date.getDayOfYear()  -  input.Loan_Date.getDayOfYear());

The result:

input.Number_of_Days = -303

I have run a few test data and it seems this bug occurs whenever the year are different. If it's the same year then the getDayOfYear function works Ok.