[LEAPSECS] Time math libraries, UTC to TAI

[Steve Summit]

Warner Losh wrote:
> On Wed, Jan 4, 2017 at 7:15 PM, Steve Summit <scs+ls at eskimo.com> wrote:
> > 2. Have xtime keep TAI.  This has the advantage that it's not at
> >    all wrong or kludgey to represent TAI as a simple count of
> >    seconds since the epoch, which of course xtime already is.
>
> The problem here, not listed, is with external stuff. When I touch
> a file, the time needs to be stored in UTC time so TAI needs to be
> converted to UTC

Sure.  But it's a given that if we keep TAI internally, we have
a way to easily convert it to UTC any time anybody asks for it.
So we do the same conversion any time we need an updated
timestamp for a file.  In general, every updated file timestamp
comes from the equivalent of clock_gettime(CLOCK_UTC) /
getimeofday() / time().

> and back again when dealing with files on disk.

Not sure what you mean there.

> It then becomes an interesting question: do you have to back convert
> form UTC to TAI when doing a stat on a file in this scheme?

No, not at all.  The timestamp in the inode on disk was stored in
UTC, as it always has been.