If you want to display time difference like twitter,facebook using PHP function then use following function which outputs time difference like 20 minutes ago,2 days ago,1 week ago

function time_difference($date)
{
if(empty($date)) {
return "No date provided";
}
$periods         = array("second", "minute", "hour", "day", "week", "month", "year", "decade");
$lengths         = array("60","60","24","7","4.35","12","10");
$now             = time();
$unix_date         = strtotime($date);
// check validity of date
if(empty($unix_date)) {   
return "Bad date";
}
// is it future date or past date
if($now > $unix_date) {   
$difference     = $now - $unix_date;
$tense         = "ago";
} else {
$difference     = $unix_date - $now;
$tense         = "from now";
}
for($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] {$tense}";
}

call this function as

$date = "2010-07-16 17:45";
$result = time_difference($date); // 1 year ago