[177495] in North American Network Operators' Group
Re: REMINDER: Leap Second
daemon@ATHENA.MIT.EDU (Valdis.Kletnieks@vt.edu)
Sun Jan 25 13:15:57 2015
X-Original-To: nanog@nanog.org
To: "John Levine" <johnl@iecc.com>
In-Reply-To: Your message of "25 Jan 2015 17:29:25 +0000."
<20150125172925.9654.qmail@ary.lan>
From: Valdis.Kletnieks@vt.edu
Date: Sun, 25 Jan 2015 13:15:27 -0500
Cc: nanog@nanog.org
Errors-To: nanog-bounces@nanog.org
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On 25 Jan 2015 17:29:25 +0000, "John Levine" said:
> It shares with time zones the problem that you cannot tell what
> the UNIX timestamp will be for a particular future time. If
> you want to have something happen at, say, July 2 2025 at 12:00 UTC
> you can guess what the timstamp for that will be, but if there's
> another leapsecond or two, you'll be wrong.
It shares another problem - that doing calculations across a boundary is
difficult. If you have a recurring timer that pops at 23:58:30 on June 30,
and you want another one in 2 minutes. do you want a timer that the next pop
is at 00:00:30 - or 00:00:29? The operating system can't tell whether the
desired semantic is "as close to every 120 elapsed seconds as possible" or
"as close to the half-minute tick as possible".
And of course doing interval math across several years where you cross multiple
leap seconds is even more problematic - for some corner cases that have an
endppoint nearmidnight, doing a naive "timestamp in seconds +/- 86400 * number
of days" can land you on the wrong *day*, with possibly serious consequences...
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