Re: Julian Dates package?

[dewar@merv.cs.nyu.edu (Robert Dewar)]

Keith said

"> That's not guaranteed to work.  The subtraction operation returns a
> result of type Duration, which is a fixed-point type with a minimum range
> of -86400.0 .. +86400.0 (i.e., +/- 1 day).  Thus, you can't portably
> subtract two Calendar.Time values more than a day apart."

This is not just theoretical. In all versions of GNAT, we use a 64-bit
value for both time and duration, so any possible period is easily covered,
but other compilers (in particular Object Ada, and maybe others) use a
32-bit representation for Duration, and the range is therefore much closer
to the required minimum (which was chosen precisely so that a 32-bit type
is somewhat usable).

We decided in GNAT to use a 64-bit type, because we don't want to have
to compromise on either the range or precision (in GNAT Duration'Small
is in nanoseconds), and the 32-bit type requires both the range and
precision to be crunched. 

But code that assumes a large range is definitely not necessarily portable,
though it will port from one version of GNAT to another!