anchor.diff:      var time = now();
       var m = (time.month + months - 1) % 12 + 1;
       var y = time.year + (months + time.month - 1) ~/ 12;
  -    var d = time.day.clamp(1, _daysInMonth(y, m));
  +    var d = _clampDate(time.day, y, m);
       return new DateTime(
           y,
           m,
  
positive.code:   DateTime yearsAgo(int years) {
      var time = now();
      var y = time.year - years;
      var d = time.day.clamp(1, _daysInMonth(y, time.month));
      return new DateTime(
          y,
          time.month,
  
negative.code:     var time = now();
      var m = (time.month - months - 1) % 12 + 1;
      var y = time.year - (months + 12 - time.month) ~/ 12;
      var d = _clampDate(time.day, y, m);
      return new DateTime(
          y,
          m,
  