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

[Joseph Gwinn]

On Sat, 18 Jan 2014 11:37:36 +0000, Zefram wrote:

> Brooks Harris wrote:

>>                                          The whole purpose of TAI is

>> a "realization" of TT, right? TAI shields us (I mean us normal

>> computer people, not astronomers or cosmologists) from the details of

>> how TAI is maintained

> 

> TAI does not shield you from the lack of atomic clocks prior to 1955.

> 

> [NTP and POSIX time]

>> I don't think so. Both are indeed counts of Seconds, and both have a

>> relationship to UTC time.

> 

> For POSIX, see

> <http://pubs.opengroup.org/onlinepubs/009604499/basedefs/xbd_chap04.html>,

> section 4.14, defining "Seconds Since the Epoch" as

> 

>     A value that approximates the number of seconds that have elapsed

>     since the Epoch.  A Coordinated Universal Time name (specified in

>     terms of seconds (tm_sec), minutes (tm_min), hours (tm_hour), days

>     since January 1 of the year (tm_yday), and calendar year minus 1900

>     (tm_year)) is related to a time represented as seconds since the

>     Epoch, according to the expression below.

>     [...]

>     tm_sec + tm_min*60 + tm_hour*3600 + tm_yday*86400 +

>     (tm_year-70)*31536000 + ((tm_year-69)/4)*86400 -

>     ((tm_year-1)/100)*86400 + ((tm_year+299)/400)*86400

>     [...]

>     The divisions in the formula are integer divisions; that is, the

>     remainder is discarded leaving only the integer quotient.

> 

> It's defined as a transformation of a broken-down UTC timestamp, not

> (despite its name) as a count of seconds since some instant.


No.  If your poke around into how time is used, you will discover that 
what is stored in the cound of seconds since the Epoch.  Broken-down 
time is used only when there is a human to be humored.



> NTP's definition, by contrast, does speak of counting seconds, but it

> doesn't count leap seconds.  It counts 86400 per UTC day regardless of

> leaps, and so is also effectively just a transformation of broken-down UTC

> timestamps.  Both NTP and POSIX time values are sometimes described as a

> "count of non-leap seconds", which similarly gets across the essential

> point that the leap second history doesn't influence the scalar<->UTC

> relationship.


POSIX time is defined without reference to NTP, which is its own world 
with its own standard.  Note that the NTP standard, RFC-1305, is dated 
March 1992, which is well after the first POSIX standard (1988 - the 
Ugly Green Book).  Nor does NTP have any reference to UNIX or POSIX.


Joe Gwinn