Add days to a specific date. Using a various groups of days

I figured out ( no let me rephase that) I used someone elses code to complete what I needed

(function () {

    // Get date from field

    var v = this.getField("Date_Completed").value;

    if (v) {

        // Convert string to date

        var d = util.scand('dd/mmm/yyyy', v);

        // Add number of days in NUMDAYS field

        d.setDate(d.getDate() + +getField("NUMDAYS").value);

        // Set value of this field to the new date

        event.value = util.printd("dd/mmm/yyyy", d);

    } else {

        // Blank field if no date entered

        event.value = "";

    }

})();

Date and time calculations in JavaScript are tricky. See these tutorials about the subject:

https://acrobatusers.com/tutorials/working-with-date-and-time-in-acrobat-javascript

https://acrobatusers.com/tutorials/working-with-date-and-time-in-acrobat-javascript-part-2

https://acrobatusers.com/tutorials/working-with-date-and-time-in-acrobat-javascript-part-3

I've written a script that allows you to easily set up this kind of calculation, without having to write any code. It uses fixed numbers, but can be easily adjusted to use the value of a field. You can find it here: Custom-made Adobe Scripts: Acrobat -- Apply Automatic Date Calculation