The libraries at momentjs.com is pretty cool. But if you just wanted something simple to calculate the "date difference" between two time values, there is a relatively simple way to do it. I assume you wanted it done in Javascript.
All you need to do is to clear out the hour/minute/second/millisecond fields of the two dates, calculate their differences in days. Put the script below in any web browser and you'll see how it works.
<script>
function foo() {
var d1 = new Date(2013, 8, 12, 13, 40, 1, 333); // any date value, last 4 params can be anything
var d2 = new Date(2013, 9, 3, 11, 42, 32, 533);
d1.setHours(0); d1.setMinutes(0); d1.setSeconds(0); d1.setMilliseconds(0);
d2.setHours(0); d2.setMinutes(0); d2.setSeconds(0); d2.setMilliseconds(0);
daysLeft = (d2.getTime() - d1.getTime())/(24*60*60*1000);
alert('Dear customer, there is(are) ' + daysLeft + ' day(s) left on your order!' );
}
</script>
<a href="#" OnClick="foo()">Show Remaining Days on Order</a>
EDIT: adding PHP version
<?php
$d1 = New DateTime('2013-08-28 06:25:00');
$d2 = new DateTime(); // now
$drip = 55;
$interval = $d2->diff($d1); // time difference
$days_left = $drip - $interval->format('%a'); // in days, subtract from drip
echo "There are $days_left days left\n";
?>