src/ResponseFactory.php
private function getModTimes(MessageInterface $request, ImageResource $resource)
    {
        $time = time();
        return [
            $time,
            (new DateTime)->setTimestamp($modDate = $resource->getLastModified()),
            strtotime($request->getHeaderLine('if-modified-since')) ?: $time,
            $modDate
        ];
    }
private function getModTimes(MessageInterface $request, ImageResource $resource)
    {
        $time = time();
        return [
            $time,
            (new DateTime)->setTimestamp($modDate = $resource->getLastModified()),
            strtotime($request->getHeaderLine('if-modified-since')) ?: $time,
            $modDate
        ];
    }
getModTimes

@param MessageInterface $request
@param ImageResource $resource

@return array