code: public static VietnameseCalendar ofTet(int gregorianYear) {
  
          return VietnameseCalendar.of(EastAsianYear.forGregorian(gregorianYear), EastAsianMonth.valueOf(1), 1);
  
      }
docs: /*[deutsch]
  <p>Erzeugt ein neues vietnamesisches Kalenderdatum am Neujahrstag (Tet). </p>
  
  @param   gregorianYear   gregorian calendar year
  @return  new instance of {@code VietnameseCalendar}
  @throws  IllegalArgumentException in case of any inconsistencies