class date has
    function this(year, month, day) do
        this.@set(year, month, day)
    end
    
    function @set(year, month, day) do
        if month > 12 do
            year += (month - 1) / 12
            month = (month - 1) % 12 + 1
        end
        
        while day > (ndays = this.@ndaysofmonth(year, month)) do
            day -= ndays; month += 1
            if month > 12 do
                month -= 12; year += 1
            end
        end
        
        while day < 1 do
            month += 1
            if month < 1 do
                month += 12; year -= 1
            end
            day += this.@ndaysofmonth(year, month)
        end
        
        this.@year, this.@month, this.@day = year, month, day
    end
    
    function @ndaysofmonth(year, month) do
        if month == 2 then
            if year % 4 == 0 and (year % 100 != 0 or year % 400 == 0) then
                29 # leap year
            else
                28 # non-leap year
            end
        elif month in [4, 6, 9, 11] then
            30
        else
            31
        end
    end
    
    function .year do this.@year end
    function .month do this.@month end
    function .day do this.@day end
    
    function operator +=(rhs) do
        this.@set(this.@year, this.@month, this.@day + rhs)
        return this
    end
    function operator -=(rhs) do
        this.@set(this.@year, this.@month, this.@day - rhs)
        return this
    end

    function operator +(rhs) do
        tmp = this.clone()
        return (tmp += rhs)
    end
    function operator -(rhs) do
        tmp = this.clone()
        return (tmp -= rhs)
    end
    
    function tostring do
        "%04d-%02d-%02d" % [this.@year, this.@month, this.@day]
    end
end