code: public SubscriptionQos setValidityMs(final long validityMs) {
          if (validityMs == -1) {
              setExpiryDateMs(NO_EXPIRY_DATE);
          } else {
              long now = System.currentTimeMillis();
              this.expiryDateMs = now + validityMs;
          }
          return this;
      }
docs: Set how long the subscription should run for, in milliseconds.
  This is a helper method that allows setting the expiryDate using
  a relative time.
  
  @param validityMs
  is the number of milliseconds until the subscription will expire
  @return the subscriptionQos (fluent interface)