[LEAPSECS] Some definitions --> practically stated

[Warner Losh]

> If it helps, perhaps we can revisit some terminology....

>

>

> NTP Time

>

> NTP time is a 32-bit logical second count since 1900 (with a 32-bit

> subsecond part) with 86400 logical seconds per day. On the leap second,

> NTP time subtracts by 1 full second - i.e. it has a step jump of 1 full

> second *back* in time. NTP time tracks GPS time less the number of leap

> seconds.


NTP time encodes the number of actual seconds minus the postive leap 
second less the negative leap seconds.  During a leap second, the ntp 
protocol encodes an extra bit to know if a leap second is happening.


> GPS time

>

> GPS time is Olson right time less 9, and TAI less 19. GPS time has an

> extra field for the current leap second count since 1980 (when there

> were only 9 leap seconds).


Not exactly.  GPS time accumlates seconds since 1980 in the form of 
truncated weeks (week mod 1024) and time of week.  When converted to 
human readable form, it is 19 seconds behind TAI time.  The GPS system 
also distributes an almanac that allows conversion of this time to a UTC 
time (a leap second count, modulo 256, and a future leap second count, 
modulo 256).

Unix systems that run on GPS time and NTP servers that serve GPS time 
appear offset by TAI - UTC + 9 seconds from systems that run on UTC time.


> TAI time

>

> TAI time is the number of SI seconds since 1970 plus 10. TAI is Olson

> right time plus 10. TAI is also a second count, but unlike Olson time,

> its precision is arbitrary.


TAI time isn't a count of seconds since 1970.  It is a time that ticks 
in 60 second minutes and 86400 second days.  Seconds in this time are 
labeled using the 'broken down' notation of YYYY-MM-DD hh:mm:ss.xxxxxx 
(or sometimes MJD hh:mm:ss.xxxxx).  There's no count here like there is 
in time_t that's converted to human readable form.  All times are 
expressed in full human readable form (although some libraries have 
encodings for this time, there's nothing defined by bipm for doing that, 
and I'm aware of at least 3 different implemenations).

And it would be more proper to say that Olson time is TAI time with an 
offset since TAI is the basis of all time definitions, and Olson time is 
just a bunch of computer software.


>

> UTC-SLS

>

> UTC-SLS is Unix time without a 1 second pause. Instead, time adjusts

> with a well-defined gradient over a course of time near the leap second.


Warner

> -paul

>

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