When the sender believed they were typing, from their own Date.now().
Kept on the wire for compatibility, but never compared against our clock: it comes from another machine, and any skew between the two would be read as the event being that much older (or newer) than it is. Expiry is stamped on arrival instead — see TypingState.add.
Null when the server withheld the identity: the user is hiding their online status and we are not permitted to see through it.