Date::externalizeValue

static function externalizeValue(FieldType $fieldType, $context, $value)
{
	//serialized date string
	if (is_string($value) && preg_match('#(.+)s[([0-9-]+)]#', $value))
	{
		$value = static::internalizeValue($fieldType, $context, $value);
	}

	if ($value instanceof ValueDate)
	{
		return $context === 'rest' ? $value->toSystemObject()->format('c') : (string) $value->toSystemObject();
	}

	if (is_string($value) && $context === 'rest')
	{
		return date('c', strtotime($value));
	}

	return $value;
}