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JavaScript alert() advancing system time by 12ms

Posted by: admin February 12, 2018 Leave a comment


Here’s a strange one:

I was working on synchronizing a JavaScript timer with a remote server and noticed my timer (based on the Date() object) was gaining about 12ms relative to my remote server on each page refresh. I narrowed it down to the JavaScript alert() function.

I created the following test and verified with 2 synchronized clocks that my system time will advance 1 second every 83-84 page refreshes. Has anybody seen this before?

<script type="text/javascript">

I’m using IE7 on XP SP3. Please tell me I’m not just going crazy!


I’m really not concerned about the accuracy of javascript timers. The real issue is that the JavaScript alert() function call is advancing my actual system time by 12ms.



most likely you are crazy, since you are counting on JavaScript timers to be precise.

This explains a lot of that: http://ejohn.org/blog/accuracy-of-javascript-time/