[LEAPSECS] Time math libraries, UTC to TAI

[Steve Summit]

Martin Burnicki wrote:
> Steve Summit wrote (offlist):
> > In some ways I agree that running the kernel on TAI would be far
> > preferable.  I've shied away from it so as to be able to answer
> > the critics who are always worrying about disconnected systems
> > that might not have a source of TAI-UTC at boot time, or at all.
>
> If your system time is disciplined via NTP then currently the TAI/UTC
> offset may not be available since the default NTP protocol doesn't
> provide it.
>
> As said earlier, you can use tzdist, or DNS, or a future NTP extension
> field to pass down the UTC/TAI offset to clients.

Right.  I'm considering all of these.

> If the kernel runs on TAI, but the TAI time isn't synchronized anyway,
> and the TAI offset is 0, then the UTC time stored in inodes matches TAI
> time on this system, but who cares?

Right.

> BTW, AFAICS current Linux kernels increment the TAI offset when they
> insert a leap second. I wonder if that is appropriate.

Uh oh, I think I see where you're going with this...

> Initially, if the kernel's TAI offset has not been set then it is 0, so
> callers of adjtimex() can see that the offset has not yet been set.
>
> However, after a leap second the kernel increases the TAI offset to 1,
> which *might* suggest to the unexperienced user that the TAI offset is
> valid.

Definitely.  This is a very good point.

> So I think the kernel should increment the TAI offset value only if
> it has been set before, i.e. if it isn't 0

I quite agree.  I hadn't thought about this case.  I shall have
to tweak my code.  Thanks.