The date object is used to work with dates and times.
时间对象用来操作日期和时间。
Note: the date object will automatically hold the current date and time as its initial value!
注意：时间对象将自动把当前的日期和时间作为初始值！
The solution is to use the valueOf method on the Date object to see how long it has been since the start of the animation.
解决方案是使用Date对象的value Of方法确定动画开始了多长时间。
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