code: public void setGregorianChange(Date date) {
          long cutoverTime = date.getTime();
          if (cutoverTime == gregorianCutover) {
              return;
          }
          // Before changing the cutover date, make sure to have the
          // time of this calendar.
          complete();
          setGregorianChange(cutoverTime);
      }
docs: Sets the <code>GregorianCalendar</code> change date. This is the point when the switch
  from Julian dates to Gregorian dates occurred. Default is October 15,
  1582 (Gregorian). Previous to this, dates will be in the Julian calendar.
  <p>
  To obtain a pure Julian calendar, set the change date to
  <code>Date(Long.MAX_VALUE)</code>.  To obtain a pure Gregorian calendar,
  set the change date to <code>Date(Long.MIN_VALUE)</code>.
  
  @param date the given Gregorian cutover date.