code: public boolean isValid(TZID tzid) {
  
          if (tzid == null) {
              return false;
          }
  
          return !Timezone.of(tzid).isInvalid(this.date, this.time);
  
      }
docs: /*[deutsch]
  <p>Existiert dieser Zeitstempel in der angegebenen Zeitzone? </p>
  
  @param   tzid    timezone id (optional)
  @return  {@code true} if this timestamp is valid in given timezone
  @throws  IllegalArgumentException if given timezone cannot be loaded