code: private static long getGregorianEpochDay(int prolepticYear, int monthOfYear, int dayOfMonth) {
          long day = yearToGregorianEpochDay(prolepticYear);
          day += getMonthDays(monthOfYear - 1, prolepticYear);
          day += dayOfMonth;
          return day;
      }
docs: Return Gregorian epoch day from Hijrah year, month, and day.
  
  @param prolepticYear  the year to represent, caller calculated
  @param monthOfYear  the month-of-year to represent, caller calculated
  @param dayOfMonth  the day-of-month to represent, caller calculated
  @return a julian day