如果时间差是负数，可以视为一个错误，因为永远不会在延迟时间结束后，在一个更早的触发时间从队列中取得项。
If the delta is ever negative, consider that an error, as you should never get an item from the queue with an earlier trigger time, when the delay has ended.
因为队列的大小没有界限，使得添加可以立即返回，但是在延迟时间过去之前，不能从队列中取出元素。
While the queue is unbound in size, enabling adds to return immediately, one cannot take an element from the queue until the delay time has expired.
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