FW: timezones offset from what?

Paul Eggert eggert at CS.UCLA.EDU
Tue May 23 07:43:56 UTC 2006

Zefram <zefram at fysh.org> writes:

> The timezone database seems to have
> the same slightly-fuzzy semantics, because it presumes a base of UTC
> in the current era but also has data for historical eras where not only
> UTC but even UT was not defined.

True.  I had forgotten about that.

>>                            It shouldn't be needed for civil times
>>anywhere on the planet,
> I think it is, where legal time is required with sub-second precision.

I don't know of any application that requires that.  I can easily
construct theoretical legal scenarios where the problem might come up,
but in practice I don't think it's ever a real problem that actually
requires writing software.  (If I'm wrong, I'd like to see the

For example, legal time in Britain is defined in terms of GMT, but GMT
itself is no longer defined as a standard, and hasn't been for
decades.  So if you wanted to argue that some legal event occurred
(say) on May 15 GMT but May 14 UTC, you'd have an uphill battle.

> This is an interesting philosophical question.  The Persian calendar
> particularly is troublesome because it's an observational calendar,
> where the month start isn't known until it actually happens.  You'd
> need to download new calendar data every month, and you can't do date
> calculations more than a few days ahead.  Rather like leap seconds.

Rather like the time zone database, too.  The topics are intimately
connected, since part of the time zone database is derived directly
from the Persian calendar.  If Iran brings back DST, then we'll have
the same problem again, of lack of long-term predictability.

> Have you developed any theory yet for what will happen with Martian
> timezones?

Nope.  Each landed mission on Mars has adopted a completely different
reference for time keeping.  I figure that we'll see a half dozen or
more systems before things settle down enough to develop a "theory".
In the mean time I'm willing to defer to Mars24 guys.

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