RFC: Time Library 0.1

Ashley Yakeley ashley at semantic.org
Thu Jul 7 05:35:33 EDT 2005

In article <Pine.LNX.4.60.0507061143280.24946 at hermes-1.csi.cam.ac.uk>,
 Tony Finch <dot at dotat.at> wrote:

> On Wed, 6 Jul 2005, Ashley Yakeley wrote:
> > Tony Finch <dot at dotat.at> wrote:
> >
> > > Is midnight 00:00 or 24:00?
> >
> > Midnight is just midnight. The formatTime function will show it as
> > "00:00".
> You miss my point that "midnight" is ambiguous, which is why ISO 8601
> allows two representations of it, for the start and the end of the day.

A DateAndTime which is midnight will canonically have the TimeOfDay as 
00:00:00. I don't yet have normalisation functions.

> > UT1 is necessary for historical time, since back-extending UTC is
> > pointless and back-extending TAI completely hopeless. The UT1 type is
> > really simple (ModJulianDate, a synonym for Rational that counts days),
> > so there's not much extra complication.
> It seems that the standard astronomical time scale is TT which is based on
> extending TAI, so it's very arguable whether you should use UT1 for
> historical time.

I mean "events in history", which are effectively known as 
approximations to UT1 (or actually, local mean time).

> If you are concerned about proper support for different
> time scales you should have a more comprehensive selection of conversion
> functions and tables, such as historical values of delta-T for converting
> between TT and UT1, etc.

I could have a limited amount of data for the earth's rotational 
history, but it would be bulky and useless for any time after the 
compilation time of the program. And without a known application, it's 
not clear how much accuracy would be appropriate.

Ashley Yakeley, Seattle WA

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