code: private static long getLeapYearsBefore(long prolepticYear) {
          // Some explanations are in order:
          // - First, because this calculates "leap years from 0 to the start of the year",
          // The 'current' year must be counted when the year is negative (hence Math.floorDiv(...)).
          // - However, this means that there's a negative count which must be offset for the in-century years,
          // which still occur at the "last two digits divisible by 6" (or 99) point.
          // - Math.floorMod(...) returns a nicely positive result for negative years, counting 'down', but
          // thus needs to be offset to make sure the first leap year is -6, and not -4...
          // The second line, which calculates the '99 occurrences, runs "backwards", so must first be reversed, then the results flipped.
          return 18L * Math.floorDiv(prolepticYear - 1, YEARS_IN_CENTURY) - Math.floorDiv(prolepticYear - 1, 4 * YEARS_IN_CENTURY) +
                  (Math.floorMod(prolepticYear - 1, 100) - (prolepticYear <= 0 ? 99 : 0)) / 99 + (prolepticYear <= 0 ? 1 : 0) +
                  ((Math.floorMod(prolepticYear - 1, 100) + (prolepticYear <= 0 ? 2 : 0)) / 6);
      }
docs: Get the count of leap years since Pax year 0.
  <p>
  This number is negative if the year is prior to Pax year 0.
  
  @param prolepticYear The year.
  @return The number of leap years since Pax year 0.