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: 
  bool _isLeapYear(int year) =>
      (year % 4 == 0) && ((year % 100 != 0) || (year % 400 == 0));
  
  /// Provides points in time relative to the current point in time, for example:
  /// now, 2 days ago, 4 weeks from now, etc.
  
negative.code:   DateTime yearsAgo(int years) {
      var time = now();
      var y = time.year - years;
      var d = _clampDate(time.day, y, time.month);
      return new DateTime(
          y,
          time.month,
  