Performance timeOrigin can be significantly in the past

patrickhousley — Thu, 11 Apr 2024 21:52:00 GMT

While building out some RUM observability code, I noticed that the `performance.timeOrigin` could be significantly in the past for some users. I don't know why or how this happens but I am wondering if it has anything to do with the browser preloading the page, or some other interaction with cache like bfcache. I have seen this issue in Firefox, Chrome, and Edge which leads me to believe my understanding of `performance.timeOrigin` is not right.

The assumption that I am currently working from is that `performance.timeOrigin` is the precise moment the page began loading (which the definition for is up for debate) while `performance.now()` is the milliseconds since that time. It's the latter part of that assumption that I really seem to have a problem with.

The data:

{ // Everything below is data from in-browser "Date.now": 1712869191634, // Browser time at time of execution "global.timeOrigin": 1712866335518, // Value of `performance.timeOrigin` "global.navigationStart": 1712869187273, // Value of `performance.timing.navigationStart` "localTimeDiff": -2851493, // `global.timeOrigin - global.navigationStart` "timeSincePageOrigin": 4361, // Value of `performance.now()` "originTime": 1712866335518, // Value of `window.performance.timeOrigin` "userAgentName": "Firefox", "userAgentOS": "Mac", "userAgentVersion": "124.0" }

From the data, it appears the browser context was create ~47 minutes before navigation started. What am I misunderstanding here?

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Performance timeOrigin can be significantly in the past

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