[LEAPSECS] alternative to smearing

[Steve Summit]

tvb at leapsecond.com (Tom Van Baak), 
> The February leap day is a useful analogy when describing leap seconds
> to non-technical people. But when you get into the details the two are
> vastly different.

A well-written list, thanks.  One quibble, though:

> 2) Past dates involving February can be handled with arithmetic.
> Past leap seconds are handled with leap second tables.
>
> 3) Future dates involving February pose no additional computing
> challenge. Future dates with leap seconds beyond a few months are
> simply not possible, or possible only with error bars.

The requirement for tables (and, correspondingly, the
"impossibility" of dealing with future UTC dates more than a few
months out) depends on what you're trying to do.  You can *talk*
about the dates, and you can do interval arithmetic with a
precision of days, hours, or minutes, perfectly well.  The only
thing you need those leap second tables for, I think, is
performing interval arithmetic with a precision of seconds.

In particular, if right now it's 2017-01-05T01:56:13, I can
obviously compute that (say) exactly three years from now is
2020-01-05T01:56:13, and I can even set a computer alarm to
go off then, even if neither I nor the (suitably programmed)
OS kernel that's tracking the alarm know in advance how many
leap seconds there might end up being between now and then.