[LEAPSECS] 256-week / leap seconds / in the news

[Steve Allen]

On Sun 2021-07-25T09:47:24-0700 Tom Van Baak hath writ:
> Per the GPS ICD, when dt_LS == dt_LSF there is no leap second to speak of;
> not in recent past; not in near future. The 8-bit WN and DN fields are not
> applicable to a non event.

I see this as one example of the problem of handling NULL in an API
and all the layers of software that use that API.  POSIX (and many
other interfaces, not just about time) supposes that it is always
possible for the system to give a value for the time now.  On social
media recently I saw a weather app showing a temperature of 0 degF for
a place that is not freezing.  In the observatory control software
that runs telescopes and instruments at Keck and Lick the overall
system allows the concept of NULL, but not all of the underlying
layers of code can handle NULL.  Software in general, and programmers,
should be prepared to handle the (hopefully) exceptional events when
the answer to "What time is it?"  is "I do not know" or "Here is a
value but I cannot confirm that it is valid."

For time in particular it has always been the case that the answer is
"Here is a value that you can use for now, but if you want to compare
it with the value deemed as correct by other systems and required by
regulations you will have to check back later using another
interface."

--
Steve Allen                    <sla at ucolick.org>              WGS-84 (GPS)
UCO/Lick Observatory--ISB 260  Natural Sciences II, Room 165  Lat  +36.99855
1156 High Street               Voice: +1 831 459 3046         Lng -122.06015
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