code: static boolean isStale(OmemoDevice userDevice, OmemoDevice subject, Date lastReceipt, int maxAgeHours) {
          if (userDevice.equals(subject)) {
              return false;
          }
  
          if (lastReceipt == null) {
              return false;
          }
  
          long maxAgeMillis = MILLIS_PER_HOUR * maxAgeHours;
          Date now = new Date();
  
          return now.getTime() - lastReceipt.getTime() > maxAgeMillis;
      }
docs: Determine, whether another one of *our* devices is stale or not.
  
  @param userDevice our omemoDevice
  @param subject another one of our devices
  @param lastReceipt date of last received message from that device
  @param maxAgeHours threshold
  
  @return staleness