os.uptime() 方法在几秒内返回操作系统的上线时间.
os.uptime()
注意：在Windows平台上，这个方法返回的秒值包含分数，请用Math.floor()获取整数值
Math.floor()
The result of this function no longer contains a fraction component on Windows.
Added in: v0.3.3
The os.uptime() method returns the system uptime in number of seconds.