JS date lib - dateFns

https://github.com/date-fns/date-fns

size: 69 kb

This is a test with date-fns

<script src="https://cdnjs.cloudflare.com/ajax/libs/date-fns/1.30.1/date_fns.min.js"></script>
<script>
var result = dateFns.differenceInDays(
        new Date(date1),
        new Date(date2)
    )
document.querySelector('#insert').innerHTML = result;
</script>