Re: [art] Predictable Internet Time

[Joe Touch <[e-mail]>]

On 3/29/2017 8:13 AM, Philip Homburg wrote:
>> Right - I will update the doc accordingly. AFAICT, POSIX time implies
>> (but never states) that it uses a local reference clock to emulate TAI.
> Not TAI, UTC.

POSIX doesn't include leaps. It can't track UTC.

Maybe we're talking across things -

POSIX defines only one internal clock - the Unix one that starts at the
Unix epoch and counts up.

time_t is supposed to return an integer of this count

If you're referring to a time_t structure that indicates days, weeks,
months, etc., you're talking about a *conversion* that *approximates*
UTC, but really basically converts the Unix clock as close as it can to
TAI and ignores the leaps.

> Every time_t value has a specific interpretation as a UTC timestamp.
There is an intended correlation between Unix dates and UTC dates, but
it isn't exact - the equation is indicated as approximate.

>  Obviously,
> without correction a free running clock will go wrong with leap seconds. But
> that doesn't change that it is some approximation of UTC and not TAI.
POSIX says it doesn't track leaps, which means (at best) it emulates
TAI, not UTC. Even if the documentation of the time_t structure and
commands indicate otherwise...

> By and large, current Unix systems have no idea about TAI. There are no 
> (commonly available) APIs that give you TAI.

The time_t struct is a conversion of the Unix count since epoch into a
"UTC-like" format, but it really is much more an emulation of TAI than UTC.

But it *is* really neither, because it isn't sync'd to anything.

Joe