docs: long calculateFirstDayOfYearMillis(int year) {
          // Java epoch is 1970-01-01 Gregorian which is 1686-04-23 Coptic.
          // Calculate relative to the nearest leap year and account for the
          // difference later.
  
          int relativeYear = year - 1687;
          int leapYears;
          if (relativeYear <= 0) {
              // Add 3 before shifting right since /4 and >>2 behave differently
              // on negative numbers.
              leapYears = (relativeYear + 3) >> 2;
          } else {
              leapYears = relativeYear >> 2;
              // For post 1687 an adjustment is needed as jan1st is before leap day
              if (!isLeapYear(year)) {
                  leapYears++;
              }
          }
          
          long millis = (relativeYear * 365L + leapYears)
              * (long)DateTimeConstants.MILLIS_PER_DAY;
  
          // Adjust to account for difference between 1687-01-01 and 1686-04-23.
  
          return millis + (365L - 112) * DateTimeConstants.MILLIS_PER_DAY;
      }