Re: Predictable Internet Time

Tony Finch <dot@dotat.at> Wed, 04 January 2017 00:00 UTC

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From: Tony Finch <dot@dotat.at>
To: Joe Touch <touch@isi.edu>
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Date: Wed, 04 Jan 2017 00:00:21 +0000
Subject: Re: Predictable Internet Time
Archived-At: <https://mailarchive.ietf.org/arch/msg/ietf/LFhjPrK2xk06MAzSA5FamSGjwDg>
Cc: Phillip Hallam-Baker <phill@hallambaker.com>, IETF Discussion Mailing List <ietf@ietf.org>
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Joe Touch <touch@isi.edu>; wrote:

>

> The Posic spec describes how to convert seconds since epoch to UTC; it
> is that conversion where leap seconds come into play.



No, it describes the reverse, how to convert broken-down UTC time to
seconds since the epoch. It multiplies minutes by a fixed 60s, hours by
a fixed 3600s, and days by a fixed 86400s. The rest of the formula is
dealing with Gregorian dates. There is no allowance for leap seconds.


http://pubs.opengroup.org/onlinepubs/9699919799/basedefs/V1_chap04.html#tag_04_16


> > But if the count of seconds includes the leap second, surely the

> > number representing the leap second could be printed properly
> > as :60 ?
>

> UTC defines it that way, so if you want to output the correct
> UTC value,
> that is the only solution.



UTC does not define a seconds count, it is specified in terms of broken-
down time only, so that doesn't really answer my question about why you
say a system can't print a :60 UTC time when you say it has a seconds
count which includes leap seconds.


There's also a relevant example in the NTP RFC



https://tools.ietf.org/html/rfc5905#page-14



1972-01-01 00:00:00 - MJD 41317 - NTP 2272060800



1999-12-31 00:00:00 - MJD 51543 - NTP 3155587200



51543 - 41317 == 10226



(3155587200 - 2272060800) / 86400 == 10226



[note integer MJD numbers are always midnight]



No leap seconds, because each one is forgotten by NTP when the leap
indicator bits are reset.


Tony.

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