<?php
 
$dates = [
    '2016-02-22 00:20:00',
    '2016-02-25 08:45:00',
    '2016-02-25 19:10:00',
    '2016-02-25 20:00:00',
    '2016-02-26 15:55:00',
    '2016-02-28 17:10:00'
];
 
$now = time();
$past_dates = [];
 
foreach ($dates as $date) {
    $timestamp = strtotime($date);
    if ($now > $timestamp) {
  		$past_dates[] = $timestamp;
  	}
}
 
$result = 'unknown';
 
if (count($past_dates) > 0) {
    echo date('Y-m-d H:i:s', max($past_dates));
}

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