Current time - past time = human readble time

Current time - past time = human readble time

I have got a function form form SC to get human readable time.
function human_time ($time)
{
$time = time() - strtotime($time); // 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':'');
}
}
And I have time as string: 2013-09-28 20:55:42
when I call this function human_time('2013-09-28 20:55:42')
then it return nothing, why ? I have added strtotime in above function.
Please tell me what is wrong.