> On Mar 12, 2015, at 3:57 PM, Tom Van Baak <[email protected]> wrote: > > Brooks, > > A couple more comments on your questions. > >> Many timekeeping systems seem to be designed for only indicating "now" >> counting forward, including NTP, POSIX, and PTP, taking short-cuts to >> avoid supplying full Leap Second and local-time metadata. > > I'm not clear why you call that a "short-cut". It's just how clocks works. > They tick forward and there is no requirement that they keep a record of time > in the past. Furthermore, any clock keeping UTC has no need for local time > metadata. So you should not lump the tz mess into the simplicity of keeping > UTC. > > The only thing a UTC clock requires is advanced notice of the length of the > current minute. This is required by no later than second 58 in the minute. > But for practical reasons a UTC clock typically gets more notice than that, > as you know. > >> I've never >> been able to understand why that practice persists despite the obvious >> need to be able to fully represent the entire post-1972 UTC timescale. >> The policy and forms of the announce signals and Leap Seconds table are >> obvious missing links, and its regrettable no official attempt has been >> made since 1972 to rectify those inadequacies. > > I don't know what you mean by represent the entire post-1972 timescale. Or > why such a need is "obvious". > > A clock does not need to represent the infinite past, present, and future of > a timescale. In the case of UTC the near future is unknowable anyway. The > present is the requirement. And the past may or may not be a requirement > depending on the user. Certainly a stand-alone RTC or time code generator or > data logger or cesium clock keeping UTC does not need to know the past. So a > historical table is not important. Only the leap second notification is > needed. > > That's why the time codes you see broadcast, like WWVB or GPS only include a > leap second notification and not a full table. > > By the way, the downside of WWVB's format is that it is not possible to > obtain TAI. With GPS, at least, TAI is not only possible but easier and more > reliable than UTC.
A clock doesn’t need to know its past. But a time scale is more than just how many seconds the current minute will have. It has a history and to compute elapsed time in that time scale, you need to know its history. Warner
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