Re: RFC: Time Library 0.1

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.

> 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. 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. Of course your software will become obsolete when
the IAU/IERS/etc. change the basis of their time scales, which happens
quite frequently.

Tony.
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