day to YMD.dart

List<int> daysToYearMonthDay(int days) {
  // Use 1st of March year -99 as base.
  // This keeps the leap day of year 0, 400, ... in the first 
  // century, but place the Feb 29 day of the century at the very end.
  days += 306 + 365 * 99 + 24;  
  
  const daysIn4Centuries = 365 * 400 + 97;
  // Multiply by 4, then divide by days in 400 years. This is basically
  // the same as dividing by 365.2425 * 100 - the average number of days
  // in a century, but using only integer division.
  var daysX4 = days * 4;
  var centuries = daysX4 ~/ daysIn4Centuries;
  var daysInCenturyX4 = daysX4.remainder(daysIn4Centuries);
  if (daysInCenturyX4 < 0) { 
    daysInCenturyX4 += daysIn4Centuries; 
    centuries -= 1; 
  }  // Don't have div compatible with %, so this is necessary for negative values.
  daysInCenturyX4 &= ~3;
  
  // Add a leap year in front, so the century + 1y starts and ends with a 
  // leap year (unless the last year is divisible by 100, but all previous
  // years always have the same lengths: 366, 365 * 3, repeat).
  daysInCenturyX4 += 366 * 4;
  const daysIn4Years = 365 * 4 + 1;
  var year = (centuries - 1) * 100 + daysInCenturyX4 ~/ daysIn4Years;
  daysInCenturyX4 = daysInCenturyX4.remainder(daysIn4Years);
  var daysInYear = daysInCenturyX4 ~/ 4;
  
  // The 5-year periods March-July and August-December both have 153 days
  // divided as 31-30-31-30-31.
  // The third period, January-February, starts with 31-x as well,
  // so all three periods can be treated equally.
  var fiveMonths = daysInYear ~/ 153;
  var daysIn5Months = daysInYear.remainder(153);
  var daysIn5MonthsX2 = daysIn5Months * 2;
  var month = daysIn5MonthsX2 ~/ 61;
  var dayInMonth = daysIn5MonthsX2.remainder(61) ~/ 2;
  month += 2 + fiveMonths * 5;
  // Correct for having rotated January-February to end of year, 
  // all the previous months really belong to the preivous year.
  // This is 1 if month is January or February, 0 otherwise.
  int isJanFeb = fiveMonths >> 1;
  month -= isJanFeb * 12;
  year -= isJanFeb ^ 1;
  return[year, month + 1, dayInMonth + 1];
}