Re: ada calendar

[Thomas Wolf <t_wolf@angelfire.com>]

rieachus@comcast.net wrote:
> The latest version of the proposed additions for Ada 0Y can be found at:
> http://www.ada-auth.org/cgi-bin/cvsweb.cgi/AIs/AI-00351.TXT?rev=1.2>

I see that this proposal includes some functionality for dealing with
leap seconds. It proposes (among other things)

procedure Difference (Left, Right : Time;
                      Days : out Day_Count;
                      Seconds : out Duration;
		      Leap_Seconds : out Leap_Seconds_Count);
    Returns the difference between Left and Right.
    Days is the number of days of difference, Seconds is the
    remainder seconds of difference, and Leap_Seconds is the number
    of leap seconds.
    If Left < Right, then Seconds <= 0.0, Days <= 0, and
    Leap_Seconds <= 0.
    Otherwise, all values are non-negative.

How is this supposed to work if Left or both Left and Right are in
the future? You cannot predict when the next leap second will be
inserted -- it depends on the irregularities of Earth's rotation,
and IERS inserts a leap second whenever necessary (well, at the end
of June or December) to keep the difference between UT1 und UTC <
0.9 sec.

Also implementing this for past dates needs a look-up table.

So, what is the number of leap seconds between 2004-01-01, 00:00:00
and 2105-01-01 00:00:00? Executing the proposed subprogram now would
probably return Leap_Seconds = 0, but executing it in 20 years, it
would probably have to return a non-zero value. (Because by then,
IERS will most probably have inserted at least one additional leap
second.)

-- 
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Thomas Wolf                          e-mail: t_wolf@angelfire.com