setting generated key expire time.

I see that in the Firefox implementation peerConnection.generateCertificate() takes an expires parameter:
Such that
 var creq = mozRTCPeerConnection.generateCertificate(
 {name: "ECDSA", namedCurve: "P-256", expires: 365*24*60*60*1000*1000 })
 .then(function (s){ console.log(new Date(s.expires)); });
creates a cert that is valid for a year.

However, this isn’t described in the the current editors draft (https://w3c.github.io/webrtc-pc/#widl-RTCPeerConnection-generateCertificate-Promise-RTCCertificate--AlgorithmIdentifier-keygenAlgorithm <https://w3c.github.io/webrtc-pc/#widl-RTCPeerConnection-generateCertificate-Promise-RTCCertificate--AlgorithmIdentifier-keygenAlgorithm>) Am I missing something ?

Tim.