How to write functions written in the decision manager

Question

I have drools customs functions , i can call the functions in my rules , i am able to call the functions sucessfully with in the rule.

The question is how to call the functions in ca test class to write unit tests

function long **f_daysBetween**(LocalDate thisDate) {
    LocalDate today = LocalDate.now();
    return ChronoUnit.DAYS.between( thisDate,today);
}