新增于: v8.5.0
Node.js 事件循环开始的高解析度毫秒时间戳。
如果事件循环尚未开始（例如，在主脚本的第一个滴答中），则该属性的值为 -1。
Added in: v8.5.0
The high resolution millisecond timestamp at which the Node.js event loop
started. If the event loop has not yet started (e.g., in the first tick of the
main script), the property has the value of -1.