[LEAPSECS] Common Calendar Time (CCT) -Brooks Harris

[Zefram]

Brooks Harris wrote:

>The idea behind "CCT" is to better define "civil time".


That seems only vaguely related to your more clearly stated objectives
of proleptic versions of TAI and modern UTC.  It's too late to better
define pre-1972 civil time, and proleptic extension of UTC doesn't affect
current civil time at all.


>The mapping between TT and TAI is known,


You're glossing over a lot here.  I'm not sure what you're trying to imply
by this statement.  It sounds rather as though you're still conflating
these two classes of time scale.


>can define unambiguous Second offsets to existing timescales that

>have origins predating 1972 where that is possible. This is

>especially important where the POSIX "the Epoch" and the NTP "prime

>epoch" are concerned.


Those are totally unimportant, as applications of proleptic UTC.  The NTP
epoch, and to some extent the Unix epoch, is not really a specific point
in time, it's merely a notional timestamp.  Both NTP and POSIX time
values are simple mathematical transforms of broken-down UTC timestamps,
not counts of seconds.  You can describe the NTP epoch as "1900-01-01
00:00:00 UTC", but all that means is that the mathematical transform maps
between that timestamp and time value 0.  That timestamp doesn't have to
be meaningful as a time (as indeed it isn't), because the only NTP time
values that actually get processed are much higher values corresponding
to contemporary times, for which UTC timestamps are meaningful.  There is
no value in associating the NTP epoch with a specific instant in time.


>from "Uniix time" or "POSIX time". (By the way, its said these are

>the same, but I don't know of any official statement to that effect.


POSIX time is a subtype of Unix time.  The term "Unix time" refers to
how time has been handled in the Unix tradition as a whole.  "POSIX
time" refers specifically to the definition in the POSIX.1 standard.
POSIX refers to UTC by name; prior to POSIX the equivalent reference in
the documentation was to GMT.


>A central problem is the definition of the POSIX "The Epoch".


Though the POSIX definition of the epoch has a problem by referring
to UTC for a time prior to the modern form of UTC, as with the NTP
epoch this is of no importance at all.  It has absolutely no impact on
the interpretation of timestamps resulting from the POSIX definition
of time_t.  Unlike NTP, there probably are Unix timestamps that old,
but since they're firmly pre-POSIX their interpretation must be governed
by the pre-POSIX traditions, and of course sub-second accuracy is not
expected for that era.

Also, if you define something that amounts to a proleptic UTC, that
doesn't in itself affect the interpretation of NTP or Unix time values.
You are not changing UTC, nor discovering previously-unknown historical
behaviour of UTC.  You are defining something new.  It would only affect
NTP if the NTP protocol were revised to explicitly reference your new
time scale in place of UTC.  That would be rather pointless, since, as
discussed above, NTP isn't used to process such historical timestamps.
A corresponding POSIX redefinition would have slightly more applicability,
as time_t does get used for historical purposes, but it still wouldn't
affect time values actually recorded in 1970.

Defining a proleptic extension of UTC is a valid and interesting exercise,
and has potential applications, but not the ones you seem to expect.
You're also mixing concerns that are better kept separate.  Proleptic TAI,
proleptic UTC, and local time zones are all more or less separate issues.

-zefram