[154428] in North American Network Operators' Group
Re: F-ckin Leap Seconds, how do they work?
daemon@ATHENA.MIT.EDU (Owen DeLong)
Tue Jul 3 19:51:32 2012
From: Owen DeLong <owen@delong.com>
In-Reply-To: <34ebe2600b91d04f9c049d59eb7e13da@mail.dessus.com>
Date: Tue, 3 Jul 2012 16:48:14 -0700
To: Keith Medcalf <kmedcalf@dessus.com>
Cc: "nanog@nanog.org" <nanog@nanog.org>
Errors-To: nanog-bounces+nanog.discuss=bloom-picayune.mit.edu@nanog.org
On Jul 3, 2012, at 1:08 PM, Keith Medcalf wrote:
>=20
>>> The system clock needs to be UTC, not UTC =B1 some offset stuck
>>> somewhere that keeps some form of running tally of the current leap
>>> second offset since the epoch.
>=20
>> Nope. UTC *includes* leap seconds already. It's UT1 that does not.
>=20
>> Are you suggesting that NTP timekeeping should be based on UT1?
>=20
> The system clock should be based on UT1 and should be monotonically =
increasing since this matches the common concept of time. Calculations =
done with this value are all based on it being UT1 and using the =
"common" notion of UT1 rules. The root cause of the difficulties is =
that someone decided that the system clock would not maintain "wall =
clock" time (UT1) but rather some other timebase and then "step" that =
time to keep it in sync with UT1.
>=20
It only matches the common concept of time at some particular instant. =
Over the course of several years it will become less and less aligned =
with the common concept of time.
Most people operate on the assumption that there are 86400*365.25 =
seconds per year overall and that every day is 86,400 seconds. UTC =
matches that common conception of time. UT1 does not because UT1 =
monotonically increments one second for every elapsed second of time and =
continues to drift out of synchronization with the celestial phenomena =
on which the common conception of time is based.
> NTP can keep time in UTC (or anything else) if it wants, but it should =
discipline the system clock to monotonically increasing UT1.
This will break many many currently correct applications and is not a =
change that should be undertaken lightly. Especially not if it is =
intended to fix a moderately esoteric bug in a few things that crops up =
once per decade or so.
Owen
