[Datetime-SIG] Fwd: Calendar vs timespan calculations...

[Alexander Belopolsky]

On Thu, Aug 6, 2015 at 11:11 AM, Chris Barker - NOAA Federal
<chris.barker at noaa.gov> wrote:
> M-A s note made this all a bit more clear to me: business use cases
> are a lot more concerned with the Calendar than actual elapsed time.
> On the other hand, I do scientific applications, and am far more
> concerned with accurate elapsed time.

Another point that some people often don't understand is that when
they run something like

$ TZ=UTC date
Thu Aug  6 18:28:49 UTC 2015

on their POSIX compliant computers that run ntpd to sync with the
atomic clock, they get a bona fide UTC time with all leap seconds
accounted for.  It is only when they do

$ TZ=UTC date +%s
1438885729

the number they get is *not* the number of seconds elapsed since UTC
midnight of 1970-01-01.  This number is some 26 seconds off, but how
many people care about this number?  On the other hand the value that
everyone looks at (Thu Aug  6 18:28:49 UTC 2015) is correct on POSIX
systems except for that one second that by strict UTC rules should be
displayed as 23:59:60.  (And, depending on how paranoid your system
admin is, you possibly see delayed time for a few hours before and
after the leap second insertion.)