Flatpickr and calculate date lengths

This is what I had in jquery:

function updateEndDate() {
        var dates = $startDate.selectedDates;

        if (dates.length === 0) {
            return false;
        }

        var duration = $('#duration').val();
        var startDate = moment(dates[0]);
        var endDate = null;

        if (duration.length > 0 && startDate.isValid()) {
            durationInt = duration.replace(/\D/g,'');
            durationKey = duration.includes('years') ? 'y' : 'M';
            endDate = startDate.add(durationInt, durationKey).subtract(1, "days");

            $('#end-date .date').text(endDate.format('Do MMMM YYYY')).parent().fadeIn();
        } else {
            $('#end-date .date').empty();
        }
    }

    var $startDate = $("#start_date").flatpickr({
        dateFormat: 'J F Y',
        onChange: function (selectedDates, dateStr, instance) {
            return updateEndDate();
        }
    });

But I'm moving away from jQuery and want to replicate this working in Vue instead

Actually managed to just replicate

 updateEndDate() {
                var dates = this.form.start_date.selectedDates;
                var duration = this.form.duration;
                var startDate = moment(dates);
                var endDate = null;

                if (duration.length > 0 && startDate.isValid()) {
                    let durationInt = duration.replace(/\D/g,'');
                    let durationKey = duration.includes('years') ? 'y' : 'M';
                    endDate = startDate.add(durationInt, durationKey).subtract(1, "days");
                    
                    this.endDate = endDate.format('Do MMMM YYYY');
                } else {
                    this.endDate = '';
                }
            }

Useful for anyone else and my future self ;)