So, seems like if payload has an iat property, that gets used instead of an internal timestamp value in sign().

var timestamp = Math.floor(Date.now() / 1000);
if (!options.noTimestamp) {
    payload.iat = payload.iat || timestamp;
  }

However, later on, expiresIn is added to timestamp and not to payload.iat to determine exp. Seems to me that that is wrong, although will not matter in the majority of cases?