2010/2/22 Mark J. Reed <markjr...@gmail.com>

> If the interface between Perl time and human time is going to be done
> through UTC, then I don't see the point in specifying that it's TAI
> behind the scenes.  Especially if you're not specifying the epoch.
> The number of seconds between two points in time in UTC is exactly the
> same as the number of seconds between two points in time in TAI, by
> definition. Only the labels differ.  And if, as you imply above, the
> labels will be UTC - which they pretty much have to be since that's
> what humans use - then Perl6 is using UTC, not TAI.  It's just using
> *real* UTC, not POSIX's broken idea of it that claims 24 seconds out
> of the past 40 years never happened.

Okay, most replies are in the same tone, so I'll reply just here...

The problem is with the idea that TAI or UTC are "the interface between Perl
time and Human Time", because that implies we could neglect the semantics of
the input type and store everything in a number of TAI seconds since the
epoch (after converting from the localtime to UTC) and store it like that.

DatesTimes are just not that simple, if I say "2009-01-01T00:00:00-0300" it
can't simply be converted to UTC then to TAI and be stored like that.
because that would mean a different year, and that semantic is not

That is why the Instant is not just a number, but a full specification of a
given instant. Which might say "2009-01-01T00:00:00 BRT" which happens to be
GMT-3 but a few years ago had DST (it's in the south emisphere, so summer
time), but it doesn't have it nowadays.

More importantly, an Instant is not a Duration since some epoch. Instant and
Duration are completely different beasts, for instance... a Gregorian
duration of 1 day is not the same as 86400 seconds, simply because we have
days with 23 hours and days with 25 hours. and adding a day should ignore
the number of seconds in that day. As well as adding a month should ignore
the numbers of days in the month and so on...

So, there isn't one interface between Perl time and Human time, simply
because there isn't just one Human time, so we deal with all human times...

Note that the same way we have Gregorian::DateTime and Gregorian::Duration,
we can have TAI::DateTime and TAI::Duration, Instant and Duration are just
the most abstract roles for that types (also note that TAI doesn't have
timezones, it started aligned with UTC and drifted away ever since).

So why have the duration TAI-based?

Simply because TAI is supposedly immutable as a scale, so it's predictable.
Gregorian time is not immutable and timezone definitions are not anyhow
predictable. So when you get a duration (in the precision of seconds) it
specifies an immutable amount of time. But if it's not in the precision of
seconds, then well... it can be anything...


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