Javascript error/calculation fail NaN/NaN/NaN

NaN stands for not a number, so you are most probably performing some calculations on variables that can not be coerced into numbers. Make sure that each step, you are performing calculations only on numbers.
It's difficult to say at which step this is happening without looking at your code.

Hi, thanks so much for the reply! It's just odd considering it worked for one record just fine, and this was the 4 record out of 6.
This is our code-

I'm not sure what the issue was. Maybe some internal things.
Try this:
Qualtrics.SurveyEngine.addOnReady(function () {
    let selector = "#" + this.questionId + " .InputText";
    jQuery(selector).flatpickr({
        dateFormat: "m/d/Y",
        onChange: function (selectedDates, dateStr, instance) {
            let dt = new Date(dateStr).fp_incr(29),
                year = dt.toDateString().split(" ")[3],
                day = dt.toDateString().split(" ")[2],
                month = dt.getMonth() < 9 ? "0" + (dt.getMonth() + 1) : dt.getMonth() + 1,
                end_date = [month, day, year].join("/");
            Qualtrics.SurveyEngine.setEmbeddedData("endDate", end_date);
            console.log(end_date);
        },
    });
});

Thanks! Will look into trying that. It's definitely odd, since it also worked fine for me in preview mode. Also exported the .qsf and imported as a new survey and worked just fine.

In that case I would recommend changing the name of the ED. Since it's one of the reserved ones.
Maybe that's creating the issue while running the survey, but not during the preview.

Nope. No problem here. See this link.

Flatpickr_Test.qsf