[科技] 使用寿命
... useful work有效功；有用功 useful time使用寿命，使用时间 useful value使用价值 ...
大多数想获得成功的人都利用有用的时间来写表示感谢、慰问和祝贺的私人信函。
As with all dead things, holding onto it won't keep it alive or change the fact that it's useful time has come and gone.死了的东西，坚持下去不会让它复活，或者改变它有用的时间已经来了并且过去了的事实。
If these are all examples of attempts to convert "down time" into useful time, take note of the way in which "down time" has been expanded.如果这都是将“无用时间”转化为有用的例子，就得注意“无用时间”是怎么被扩展的。
