We really can't promise much when we use the window.setInterval in JavaScript.
The WhatWG specification of timers says that the timer will restart after running the callback code, and after the current document has been "fully active" for 'timeout' milliseconds. That's too vague to make promises from.
The current VM timer actually schedules the timer at regular intervals (start, start+timeout, start+2*timeout, etc.) and fires as soon as possible after each.
That can mean that there are less than 'timeout' milliseconds between two events.

We can promise that the timer repeats, and that it repeats no more than n times in n*timeout milliseconds, but apart from that, no promises.

I'll see if I can document this somewhere.

Documentation added.

The VM and the WhatWG spec differ on how to handle repeating timers, so this is as precise as we can get.

We might want to consider changing the VM to use the same behavior as the browser, but that's another issue.

Added Fixed label.