var dateValues = [
    document.getElementById('event-start'),
    document.getElementById('event-end')
];

var formatter = new Intl.DateTimeFormat('en-GB', {
    dateStyle: 'full'
});

dateSlider.noUiSlider.on('update', function (values, handle) {
    dateValues[handle].innerHTML = formatter.format(new Date(+values[handle]));
});