- strtotime: documentation ( source)
<?php
$time1 = strtotime('2017-3-18');
$time2 = strtotime('2017-3-19');
$formater = new \IntlDateFormatter('en_US' . '@calendar=persian' ,
\IntlDateFormatter::FULL, \IntlDateFormatter::FULL, 'Asia/Tehran',
\IntlDateFormatter::TRADITIONAL, 'y-M-d');
echo $formater->format($time1), PHP_EOL;
echo $formater->format($time2), PHP_EOL;