Return server timestamp in all (or just certificate/sign) responses

I am seeing a little clock skew locally between my laptop, my Android devices, and the two servers (picl-idp and token server) that do the FxAccount authentication dance.  We'd like clients to be able to estimate local clock skew and adjust if possible, which requires the server provide its local time.  Seems like the easiest thing would be for the server to add a timestamp header (milliseconds since epoch) to all replies.  IIRC, Sync/Weave called this header X-Timestamp.

I'm going to file a token server ticket as well.