Re: HTML Time Zone proposal

Arle Lommel scripsit:

> * If we don’t use tz, we know what it means (some time defined with
> respect to UTC).

Am I correct in interpreting this as "If we don't use either tz or
an offset"?  If that has a defined meaning of UTC, then we cannot
express "5:00 PM New York time" without an offset, for specifying
"2020-04-01T17:00:00" must mean the same time whether tz is specified
or not, for the sake of downward compatibility with systems that don't
accept tz.

If on the other hand a timezone without an offset means some unspecified
offset, then using tz will work.  But I can hardly suppose that is the
case.

> * If we use tz and no offset, we know what it means (the time as
> defined by the time zone, which may refer to a future point where we
> do not yet know the offset).

How can that be downward compatible?

> The problem is what to do with combinations that have a positive
> conflict (e.g., UTC+1215 but tz=“Americas/Phoenix”).

We cannot tell in the general case what is a conflict, because any part
of the earth might change its offsets and DST rules at any time.  This
one may seem improbable, but the Philippines changed from UTC-6 to UTC+8
at the end of 1845 (hence the trick question "What happened in the
Philippines on Dec. 31, 1845?" to which the answer is "Nothing, because
the date did not exist").  Similarly, Kiribati switched from - to + time
in 2005, and there have been other cases.  So we must assume that if
both offset and tz are present, a conflict exists.

> That is still true. But if we encounter a time expressed with an
> offset (and there are lots of those already out there), we do know
> what it corresponds to in UTC and it is an entirely valid proposition
> to refer to whatever time will correspond to 14:00 UTC+2 in New York
> City on 2124-10-25. We don’t know what exact local time that will be
> yet, but we can still refer to it in your scenario.

But that is not the scenario.  If I say "Let's meet in exactly five
years at 5:00 in Grand Central Station", then I am specifying a local
time, not a universal time.  IOW, I am saying "Let us meet at whatever
universal time corresponds to this local time", not "Let us meet at
whatever local time corresponds to this universal time".

-- 
John Cowan          http://www.ccil.org/~cowan        cowan@ccil.org
I don't know half of you half as well as I should like, and I like less
than half of you half as well as you deserve.  --Bilbo

Received on Friday, 15 August 2014 15:33:12 UTC