- From: Rob Smith via GitHub <sysbot+gh@w3.org>
- Date: Mon, 24 Apr 2023 15:52:24 +0000
- To: public-sdwig@w3.org
@chris-little Thanks for your feedback. > 1. @rjksmith I suggest that you may want to add the note along the lines of: > `The Second component cannot be 60 or 61; leap seconds cannot be represented.` for the seconds. There is a note to that effect in the HTML valid global time web page. > I think this would apply to both the media start time and the cue offsets. > I am sure someone, somewhere, will record, or already have recorded, across a leap second or two introduction. Maybe a one or two second discrepancy is acceptable, but at least be explicit about it. Start and end times of [WebVMT cues](https://w3c.github.io/sdw/proposals/geotagging/webvmt/#webvmt-cue) are properties inherited from [text track cue](https://html.spec.whatwg.org/multipage/media.html#text-track-cue) which are time offsets relative to the start of the [media timeline](https://html.spec.whatwg.org/multipage/media.html#media-timeline), i.e. zero. I believe that these properties are agnostic of leap seconds. ``` NOTE There are 3 seconds before this cue starts, and 2 more seconds until it ends. 00:00:03.000 --> 00:00:05.000 ``` The WebVMT [media start time](https://w3c.github.io/sdw/proposals/geotagging/webvmt/#dfn-media-start-time) can be used to calculate the [timeline offset](https://html.spec.whatwg.org/multipage/media.html#timeline-offset) property of the media element. This links the [media timeline](https://html.spec.whatwg.org/multipage/media.html#media-timeline) to an explicit date and time, though I believe that the [media timeline](https://html.spec.whatwg.org/multipage/media.html#media-timeline) still remains agnostic of leap seconds. Do you have a counterexample? > 2. Maybe also a note to the effect that only clocks that are stationary w.r.t. to each other can be exactly synchronized. If they are moving w.r.t.each other, accuracy of synchronisation depends on the relative speeds, distances and the number of iterations of the synchronisation messages. (This is not a relativistic thing, it was a railway issue more than a century ago, finally sorted by Poincaré.) The closest reference I can find to your description is [local time](https://en.wikipedia.org/wiki/Henri_Poincaré#Local_time) which is associated with special relativity. ``` t' = t + vx/c^2 ``` However, you mentioned that this is not relativistic, so I'm unclear whether I've understood correctly. Please confirm details of the synchronisation issue (or a suitable link) and how this would affect WebVMT. Note that [WebVMT timestamps](https://w3c.github.io/sdw/proposals/geotagging/webvmt/#dfn-webvmt-timestamp) are only accurate to the nearest millisecond. -- GitHub Notification of comment by rjksmith Please view or discuss this issue at https://github.com/w3c/sdw/issues/1397#issuecomment-1520429476 using your GitHub account -- Sent via github-notify-ml as configured in https://github.com/w3c/github-notify-ml-config
Received on Monday, 24 April 2023 15:52:30 UTC