A simple Twig Filter extension for Symfony2 that converts a JSON Date object into a timestamp that can be used in the PHP date() function.

Extension.php

<?php
namespace Acme\DemoBundle\Twig;

use Twig_Extension;
use Twig_Filter_Method;

class AcmeExtension extends Twig_Extension
{
	public function getFilters()
	{
		return array(
			'parseJsonDate' => new Twig_Filter_Method($this, 'parseJsonDate'),
		);
	}

	/**
	 * Parse a date string from JSON for easier manipulation
	 * Assumed public domain - http://stackoverflow.com/a/16749812/996876
	 *
	 * @param string $date JSON date string
	 * @param string $format See http://php.net/manual/en/function.date.php
	 * @param string $calculation Perform a calculation to $date using a
	 *							  string compatible with strtotime()
	 * @return array|bool Timestamp and formatted textual representation
	 */
	public function parseJsonDate($date, $format = 'Y-m-d H:i:s', $calculation = '')
	{
		// Parse the date into timestamp string and timezone string
		preg_match('/(\d{10})(\d{3})([\+\-]\d{4})/', $date, $matches);

		// Get the timestamp as the TS tring / 1000
		$ts = (int) $matches[1];
		if ($calculation) {
			$ts = strtotime($calculation, $ts);
		}

		// Get the timezone name by offset
		$tz = (int) $matches[3];
		$tz = timezone_name_from_abbr("", $tz / 100 * 3600, false);
		$tz = new \DateTimeZone($tz);

		// Create a new DateTime, set the timestamp and the timezone
		$dateTime = new \DateTime();
		$dateTime->setTimestamp($ts);
		$dateTime->setTimezone($tz);

		// Echo the formatted value
		return $dateTime->format($format);
	}

	public function getName()
	{
		return 'acme_extension';
	}
}

services.yml

    acme.twig.extension:
        class: Acme\DemoBundle\Twig\AcmeExtension
        tags:
            - { name: twig.extension }