code: private long getISOLeapYearCount(long prolepticYear) {
          long offsetYear = prolepticYear - (end == Month.JANUARY ? 1 : 0) - 1;
          return Math.floorDiv(offsetYear, 4) - Math.floorDiv(offsetYear, 100) + Math.floorDiv(offsetYear, 400) + (end == Month.JANUARY ? 1 : 0);
      }
docs: Return the number of ISO Leap Years since Accounting Year 1.
  <p>
  This method calculates how many ISO leap years have passed since year 1.
  The count returned will be negative for years before 1.
  This method does not validate the year passed in, and only has a
  well-defined result for years in the supported range.
  
  @param prolepticYear  the proleptic-year to check, not validated for range
  @return the count of leap years since year 1.