Re: GPS Leap Second Scheduled!

[hpa@transmeta.com (H. Peter Anvin)]

Followup to:  <199809092149.RAA06993@hilfy.ece.cmu.edu>
By author:    "Brandon S. Allbery KF8NH" <allbery@kf8nh.apk.net>
In newsgroup: linux.dev.kernel
>
> In message <19980909200032.B13292@caffeine.ix.net.nz>, Chris Wedgwood writes:
> +-----
> | On Wed, Sep 09, 1998 at 12:59:47AM +0000, H. Peter Anvin wrote:
> | > The way xntp deals with leap seconds is it lets the epoch float...
> | > i.e. it holds time_t to the same value for two seconds.
> | Cool... so 1970 becomes even longer ago that I would have assumed
> | then?
> +--->8
> 
> That's the only semi-reasonable (I stress *semi-*) interpretation one can 
> give to the current standard.  The other possibilities are worse.
> 

Right.  I think the right solution is one I suggested on c.o.l.d.s
recently:

- time_t being a 64-bit signed integer linked to UTC
- struct timespec (and struct timeval, presumably) having an extra
  field added:

	64-bit seconds field (same as time_t) linked to UTC
	32-bit nanosecond field (microsecond for timeval) linked to TAI
	32-bit integral TAI-UTC difference

That way any moment in time will be uniquely derivable at least, at a
positive leap second, you will see the progression (where T means a
time_t value divisible by 86400):

	tv_sec		tv_delta	UTC

	T-3		N		23:59:57
	T-2		N		23:59:58
	T-1		N		23:59:59
	T-1		N+1		23:59:60
	T		N+1		00:00:00
	T+1		N+1		00:00:01
	T+2		N+1		00:00:02

... and for a negative leap second ...

	tv_sec		tv_delta

	T-3		N		23:59:57
	T-2		N		23:59:58
	T		N-1		00:00:00
	T+1		N-1		00:00:01
	T+2		N-1		00:00:02

We may want to have a testable bit that we're "in" a leap second for
the 23:59:60 case...

	-hpa

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