docs: function add(this_, part, value){
      var getter = GETTER[part];
  
      if (!getter)
      {
        /** @cut */ basis.dev.warn('basis.date.add: Unknown date part `' + part + '`, date not changed');
        return this_;
      }
  
      var day;
      if (part == 'year' || part == 'month')
      {
        day = this_.getDate();
        if (day > 28)
          this_.setDate(1);
      }
  
      SETTER[part](this_, getter(this_) + value);
  
      if (day > 28)
      {
        var monthDayCount = getMonthDayCount(this_);
        this_.setDate(Math.min(day, monthDayCount));
      }
  
      return this_;
    }