Re: ADA.CALENDAR and midnight

[Frank Piron <empty@zero.nil>]

Hi,

Thu, 05 Aug 2004 20:00:52 +1000  Brian May <bam@snoopy.apana.org.au> wrote:

> Hello,
>
> According to documentation I have:
>
> subtype DAY_DURATION is DURATION range 0.0 .. 86_400.0;
>
> How is midnight meant to be represented?
>
> I would have assumed it would be 0.0, the next day, but preliminary
> tests, unless I am confused, seem to show it is represented as
> 86_400.0 (using gnat compiler for Windows)? The following quote from
> the documentation would seem to imply I am correct; "SECONDS
> indicating the number of seconds past midnight", but I am getting
> behaviour which suggests otherwise.
>
> Why are both 0.0 and 86_400.0 defined to be legal values?  Aren't they
> both the same thing (but one day apart)?

Any instance of type DAY_DURATION represents a timespan.
"midnight" means a point in time (more precisely a discrite,
infinite subset of all points in time).

If you would take away 86_400.0 from DAY_DURATION then you would
have a unique representation for every point in time p as:

p = midnight + dd (dd a DAY_DURATION instance)

on the one hand. But on the other hand the timespan of a full day
could not be represented by an instance of DAY_DURATION.

-- 
Crisis? What Crisis?
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