Answers for "why php date is different when convert using php date"

convert one time zone datetime value to another using php

<?php
$date = new DateTime('2000-01-01', new DateTimeZone('Pacific/Nauru'));
echo $date->format('Y-m-d H:i:sP') . "\n";

$date->setTimezone(new DateTimeZone('Pacific/Chatham'));
echo $date->format('Y-m-d H:i:sP') . "\n";
?>
  
  2000-01-01 00:00:00+12:00
2000-01-01 01:45:00+13:45

convert date php

$var = '21/05/2012';
$date = str_replace('/', '-', $var);
echo date('Y-m-d', strtotime($date))