[LEAPSECS] stale leap second information

[Martin Burnicki]

Tom Van Baak wrote:
> Or use this 40-line text file instead:
>      http://hpiers.obspm.fr/iers/bul/bulc/Leap_Second.dat
> The one is nice because it includes a "File expires on 31 December 2015" notice.

This in not an optimal solution, yet, since parsing a string like this 
can be error prone. The approach used by the NIST format is better since 
it contains the time stamp in a machine readable format and in addition 
in a human readable format. Just my 2 cents.

And, applications evaluating the expiration date should emit a warning 
quite some time *before* the expiration date becomes current.

For example, the current versions of the files contain the leap second 
date at end of June, and the files expire at the end of December, when 
(at least in theory) another leap second might be scheduled.

If a warning for the expired file is only emitted at December 28 or 30 
then admins don't have much time to care about an update of this file. 
If they are warned earlier then they can carefully look at this, and an 
updated version of the files should be available roughly 6 months before 
the expiration of the previous one.

Martin