Countdown with Views
If you use Display Suite, you can create a custom field with the Display Suite Format and use the following (with your own date field name). You could also use a custom PHP field or some kind of hook, but the following logic still applies. 

&lt;?php
if (!empty($entity-&gt;field_event_start)) {

    // Get the starting time value and the timezone.
    $start_value = $entity-&gt;field_event_start['und'][0]['value'];
    $timezone = $entity-&gt;field_event_start['und'][0]['timezone'];

    // Create the date objects for now and the day of the event.
    $start_date = new DateObject($start_value, $timezone);
    $now = new DateObject(time(), $timezone);

    // Take the difference between now and the beginning date, in days format.
    $duration = $now-&gt;difference($start_date, 'days');
    print $duration . ' Days';
}
?&gt;


You should end of with something like the following, and then jazz it up with your own styling.



Edit

I just realized you wanted "since" as in after, so just switch it up a bit and use the end date instead of the start date, if one exists. I also made a quick video on how to add in the custom code field, so hopefully this helps!

Custom Date Since field in Display Suite