Lua中的帧独立时间？

为什么不在程序开始时或者开始的地方设定时间（starting_time = os.time()），然后'当前经过时间'就是 os.time() - starting_time。不需要累加...

function make_stopwatch ()
    local start = 0
    local finish = 0
    local function sw (cmd)
        if cmd == "start" then
            start = os.time()
            return 0
        end
        if cmd == "lap" then
            return os.difftime(os.time(), start)
        end
        if cmd == "stop" then
            finish = os.time()
        end
        return os.difftime(finish, start)
    end
    return sw
end

演示：

> sw = make_stopwatch()
> =sw("start")
0
> =sw("lap")
16
> =sw "lap"
22
> =sw "lap"
28
> =sw "stop"
42
> = sw()
42
> = sw()
42
> = sw "start"
0
> = sw "lap"
8
>