<?php

 //$postedDateTime = '2012-09-23 09:11:02';
 $postedDateTime = date('2012-09-23 09:11:02');


 function time_elapsed_since ($postedDateTime){

    $time = time() - $postedDateTime; // to get the time since that moment

         $tokens = array (
                      31536000 => 'year',
                      2592000 => 'month',
                      604800 => 'week',
                      86400 => 'day',
                      3600 => 'hour',
                      60 => 'minute',
                      1 => 'second'
                  );

                  foreach ($tokens as $unit => $text) {
                      if ($time < $unit) continue;
                      $numberOfUnits = floor($time / $unit);
                      return $numberOfUnits.' '.$text.(($numberOfUnits>1)?'s':'');
                  }

 }


    echo 'Mr. Tharanga added his tutor profile ' . time_elapsed_since($postedDateTime) . ' ago';


?>

$postedDateTime = date('2012-09-23 09:11:02');

$postedDateTime = strtotime('2012-09-23 09:11:02');

$postedDateTime = '2012-09-23 09:11:02';
     //$postedDateTime = date('2012-09-23 09:11:02');


     function time_elapsed_since($postedDateTime){
        $time = time() - strtotime($postedDateTime); // to get the time since that moment
             $tokens = array (
                          31536000 => 'year',
                          2592000 => 'month',
                          604800 => 'week',
                          86400 => 'day',
                          3600 => 'hour',
                          60 => 'minute',
                          1 => 'second'
                      );

                      foreach ($tokens as $unit => $text) {
                          if ($time < $unit) continue;
                          $numberOfUnits = floor($time / $unit);
                          return $numberOfUnits.' '.$text.(($numberOfUnits>1)?'s':'');
                      }
     }
     echo 'Mr. Tharanga added his tutor profile ' . time_elapsed_since($postedDateTime) . ' ago';

Try This

<?php
 //$postedDateTime = '2012-09-23 09:11:02';
     date_default_timezone_set('Asia/Kolkata');
     $postedDateTime = '2012-09-23 16:48:00';


     function time_elapsed_since($postedDateTime)
     {
        $start_date = new DateTime($postedDateTime);
        $time = $start_date->diff(new DateTime(date('Y-m-d H:i:s'))); // to get the time since that moment
        $timeDiff = null ;

        $timeDiff .= ($time->y) >= 1 ? $time->y .' years' : null;
        $timeDiff .= ($time->m) >= 1 ? $time->m .' months' : null;
        $timeDiff .= ($time->d) >= 1 ? $time->d .' days' : null;
        $timeDiff .= ($time->h) >= 1 ? $time->h .' hours' : null;
        $timeDiff .= ($time->i) >= 1 ? $time->i .' minutes' : null;
        $timeDiff .= ($time->s) >= 1 ? $time->s .' seconds' : null;

        return $timeDiff;
    }

        echo 'Mr. Tharanga added his tutor profile ' . time_elapsed_since($postedDateTime) . ' ago';

?>