[LEAPSECS] Civil timekeeping before 1 January 1972

[Warner Losh]

> On Mar 12, 2015, at 3:57 PM, Tom Van Baak <tvb at LeapSecond.com> 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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