This is just a simple day difference function. I wrapped the solution detailed here on StackOverflow into a function:

function day_difference($date) {

	$now = time(); // or your date as well
	$your_date = strtotime($date);
	$datediff = $now - $your_date;
	return round($datediff / (60 * 60 * 24));

}

These are all variations on a standard time ago function that will provide more detail than day difference or less detail as the day difference grows into the weeks, months and beyond. There is a different variation on this used on this site which I detail in this previous post:

function ago($time)
{
   $periods = array("second", "minute", "hour", "day", "week", "month", "year", "decade");
   $lengths = array("60","60","24","7","4.35","12","10");

   $now = time();

       $difference     = $now - $time;
       $tense         = "ago";

   for($j = 0; $difference >= $lengths[$j] && $j < count($lengths)-1; $j++) {
       $difference /= $lengths[$j];
   }

   $difference = round($difference);

   if($difference != 1) {
       $periods[$j].= "s";
   }

   return "$difference $periods[$j] ";
}

Found on this StackOverflow thread:

function time_elapsed_string($datetime, $full = false) {
    $now = new DateTime;
    $ago = new DateTime($datetime);
    $diff = $now->diff($ago);

    $diff->w = floor($diff->d / 7);
    $diff->d -= $diff->w * 7;

    $string = array(
        'y' => 'year',
        'm' => 'month',
        'w' => 'week',
        'd' => 'day',
        'h' => 'hour',
        'i' => 'minute',
        's' => 'second',
    );
    foreach ($string as $k => &$v) {
        if ($diff->$k) {
            $v = $diff->$k . ' ' . $v . ($diff->$k > 1 ? 's' : '');
        } else {
            unset($string[$k]);
        }
    }

    if (!$full) $string = array_slice($string, 0, 1);
    return $string ? implode(', ', $string) . ' ago' : 'just now';
}