code: public static int fixedFromGregorian(int year, int month, int day) {
          int yearM1 = year - 1;
          return 365 * yearM1 +
                  yearM1 / 4 -
                  yearM1 / 100 +
                  yearM1 / 400 +
                  (367 * month - 362) / 12 +
                  (month <= 2 ? 0 :
                          isLeapYear(year) ? -1 :
                                  -2) +
                  day;
      }
docs: the number of days since the <em>epoch</em>,
  which is the imaginary beginning of year zero in a hypothetical
  backward extension of the Gregorian calendar through time.
  See "Calendrical Calculations" by Reingold and Dershowitz.