PHP
$canonicalized_time = $timestamp->C14N(); 
$digest_time = base64_encode(pack("H*", hash("sha256",$canonicalized_time )));  

This gives back the correct result. I am still not sure why do they simple C14N instead of exc C14N.

The lesson should be: if it does not happen, make sure that:

1. The hex is converted to binary before base64
2. Try multiple C14N, because they sometime lie to you

Please correct me if I said anything wrong.