Event.timeStamp reveals startup time

Experimenting with JS Events, I noticed the following:

If I open a web page's console in standard Firefox and enter new Event("asdf").timeStamp it returns a number like 1442000361118206.

This corresponds to the number of microseconds since the epoch. I confimed this result by entering:

window.setInterval(() => console.log(new Event("asdf").timeStamp), 1000)

and it prints out

1442000426991504
1442000427990600
1442000428989561
1442000429989980
1442000430990465
1442000431990790
1442000432989823
1442000433989783
...

This violates the web standard, which specifies milliseconds since the epoch: http://www.w3.org/TR/2000/REC-DOM-Level-2-Events-20001113/events.html#Events-Event-timeStamp

Tor Browser gives similar result, but rounded to the nearest 100 microseconds, because of TBB's #1517 (moved) patch.

In any case, that's not the usual way to use Events. A more typical usage is:

window.addEventListener("mousedown", e => console.log(e.timeStamp))

If I click the mouse once per second (by listening to a clock) I get:

1028609
1029641
1030609
1031641
1032617
1033609
1034601
1035601
...

which indicates that the results are expressed in milliseconds. The epoch, however, is defined differently. In this case it reveals the system startup time, which is a linkability issue.