Re: Predictable Internet Time

Nico Williams <nico@cryptonector.com> Tue, 28 March 2017 17:39 UTC

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Date: Tue, 28 Mar 2017 12:39:18 -0500
From: Nico Williams <nico@cryptonector.com>
To: Stewart Bryant <stewart.bryant@gmail.com>
Cc: ietf@ietf.org
Subject: Re: Predictable Internet Time
Message-ID: <20170328173916.GE7490@localhost>
References: <CAMm+LwgfQJ8aG5wB=d3fRbbeje3J9o7Z4_DCuP8DL88ouDeKzw@mail.gmail.com> <504e2cea0d1668c31486b05fec0a967a4446aefe@webmail.weijax.net> <CAMm+Lwi_jU6gjdtdM6a2n_9_89tUvWBNXxnMtSjTEA++h1D4Ew@mail.gmail.com> <e0a43370-751f-808c-3719-9716f9cd57d1@isi.edu> <alpine.DEB.2.11.1701031348430.7102@grey.csi.cam.ac.uk> <f94415b6-d9f7-0a03-cf5b-ce39c109aa71@isi.edu> <f9429571-b9d5-75d4-9b46-b877a189a7bf@gmail.com>
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On Tue, Jan 03, 2017 at 06:35:03PM +0000, Stewart Bryant wrote:
> Yes, the system should use a leap-second-free constant-duration-seconds time
> for everything. It is only humans that need the variable jumpy version of
> time presented to them, and that is a UI issue.

I don't think that's quite right.

Unix time is canonically a count of seconds since an epoch and admits no
leap seconds.  A conversion of Unix time to adjusted (i.e., with leap
seconds added) time since the same epoch would be nice, as well as:

 - conversion from either to broken down time
    - with seconds count up to 60 when converting from seconds-with-leaps
 - formatting of dates from either as well as from broken down time
    - but when converting from seconds-with-leaps the formatted string
      can show seconds-in-minute values of 60
    - whereas when converting from Unix seconds the formatted string can
      never show seconds-in-minute values of 60
 - parsing to broken down time and to seconds-since-epoch types
    - ignoring leap seconds when converting to Unix time
    - correctly accounting for all leap seconds prior to that time when
      converting to seconds-with-leaps-since-epoch

It's all about data typing.  In POSIX C API terms, we need new functions
to deal with seconds-with-leaps-since-epoch, and obviously a lot of
prayer because there's no way in C to distinguish one integer type from
the other (well, the new type could be wrapped in a struct..).

We will have to deal with multiple kinds of time as long as different
applications/users need different definitions of time.  Astronomers may
need corrected time, while bankers may want to ignore leap seconds --
all on the same systems (same OSes, same code libraries).

Functionally this is about data typing -- preferably strong data typing.

Smearing is a third time representation that also needs conversion to
the other types (more opportunity for bugs!); it leaves a bad taste.  It
may work for some apps though, but all nodes had better smear in exactly
the same way -- within your tolerances -- if your app depends on time
for synchronization.

Nico
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