php - 无法从字符串转换日期

I want to get this date's timestamp from string. I'm trying:

DateTime::createFromFormat("Y-m-d", "2015-05-18T23:45:21.604Z");

But it returns nothing. I don't know which format is this.

try this,

$date = date_create_from_format('d/M/Y:H:i:s', '2015-05-18T23:45:21.604Z');
$date->getTimestamp();