[tz] What's "right"?

Guy Harris gharris at sonic.net
Sat Nov 14 02:25:30 UTC 2020

On Nov 12, 2020, at 7:00 PM, Brooks Harris <brooks at edlmax.com> wrote:

> See
> IEEE Std 1003.1-2017 (Revision of IEEE Std 1003.1-2008)
> A.4 General Concepts
> https://pubs.opengroup.org/onlinepubs/9699919799/xrat/V4_xbd_chap04.html
> Section:
> A.4.16 Seconds Since the Epoch

Yes, I have - many times.  (Back when I was at Sun, the question of what to do with leap seconds was under discussion; Sun was trying to get leap second support to be allowed, but we lost that battle.)

> I believe Joe Gwinn may have something to say about this section.

I don't know about Joe Gwinn, but I *do* know that Doug Gwyn was pushing the ANSI C committee to allow tm_sec to be > 59; he was successful in doing so.

But none of this addresses what I was discussing in the message to which you're responding, which is "can one speak of UTC for a date and time in 1970 and, if so, how does that relate to the current specification of UTC?"

For what it's worth, the Wikipedia page for Coordinated Universal Time:


	The coordination of time and frequency transmissions around the world began on 1 January 1960. UTC was first officially adopted as CCIR Recommendation 374, Standard-Frequency and Time-Signal Emissions, in 1963, but the official abbreviation of UTC and the official English name of Coordinated Universal Time (along with the French equivalent) were not adopted until 1967.

	The system has been adjusted several times, including a brief period where the time-coordination radio signals broadcast both UTC and "Stepped Atomic Time (SAT)" before a new UTC was adopted in 1970 and implemented in 1972. This change also adopted leap seconds to simplify future adjustments. This CCIR Recommendation 460 "stated that (a) carrier frequencies and time intervals should be maintained constant and should correspond to the definition of the SI second; (b) step adjustments, when necessary, should be exactly 1 s to maintain approximate agreement with Universal Time (UT); and (c) standard signals should contain information on the difference between UTC and UT."

Presumably 1970-01-01 00:00:00 UTC is in "the new UTC".

> UTC1972 is the initial alignment point between TAI and UTC:
> 441763210s (TAI) = 1972-01-01 00:00:10 (TAI) = 1972-01-01T00:00:00 (UTC)
> The Posix "the epoch" is defined as 1970-01-01 00:00:00 (UTC), exactly 63072000 seconds (two years, 365 days x 2 = 730 days x 86400 seconds = 63072000s) before UTC1972, here called UTC1970.

So that's 1970-01-01 00:00:00 (UTC, meaning "the new UTC, extended backwards to 1970, with no leap seconds added or removed prior to 1972").

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