Writing complex formuals in a stupid simple way is worthwhile as you may be the poor soul who 5 years from now needs to understand a formula ( and you may have been the one to write it).

Also using comments is useful. A comment is a line or part of a line beginning with //

for example

// first calculate last year same day.

var date SameDayLastYear = AdjustYear([Certification Date],-1);

// then we subtract 15 days.

$SameDayLastYear - Days(15)