[LEAPSECS] ISO 8601 Z designator improper before 1972?

Zefram zefram at fysh.org
Mon Feb 9 12:00:01 EST 2009

Gerard Ashton wrote:

>is it fair to conclude that it is improper to use the "Z" designator for any

>date/time in the ISO 8601 format prior to 1 January 1972?

That's a tricky area. We discussed the interpretation of "Z" a little in
2008-12, in a thread titled "Footnote about CCITT and UTC". I commented:

:Section 4.2.5 ("Local time and Coordinated Universal Time (UTC)")

:is about representing the difference between local time and UTC.

:It doesn't actually use the term "standard time", but clearly for the

:difference to be exact the local time must be a standard time as defined

:in section 2.1.4.


:The other representation that is defined specifically by reference to

:UTC is in section 4.2.4 ("UTC of day"). This is about the use of "Z"

:to tag a time-of-day as being on the UTC time scale.


:So, if we follow the standard as written, I can validly (in ISO

:8601) state that the BST (British Summer Time, = GMT + 1h) time is

:"22:32:12.123", but I can't write it as "22:32:12.123+01:00" or describe

:BST as "+01:00". The offset from UTC to BST is not exactly one hour,

:but some constantly-changing value that is best determined by the IERS.

:Likewise, I can write the GMT time, which is the legal time in Britain,

:as "21:32:12.123", but not as "21:32:12.123Z", because GMT is not

:precisely UTC. (I'm using "GMT" in the strict sense here, of course.)


:Unfortunately ISO 8601 does not supply any way to designate UT1 or any

:other flavour of UT except UTC. Thus, strictly speaking, there is no

:way to designate any local time that is based on UT1 rather than UTC.

:As we know from previous discussions on this list, there are quite a few

:such time scales with current legislative endorsement. (And a few that

:can't decide which they're based on.)


:It seems to me that the standard would be rather more useful if "standard

:time" were defined more according to its original meaning: a local time

:scale defined by an offset from UT, rather than specifically from UTC.

:The timezone designation material should correspondingly refer to UT

:rather than UTC, and the "Z" should probably follow by designating UT.

:All of these would be explicitly vague as to which flavour of UT is

:being referred to.


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