## 1368Re: [ISO8601] Re: Fraction of days, ISO-8601 2005

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• Sep 14, 2005
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there is a third way:

How about we slow the earth down just a smidgeon so seconds are evenly
divisible by multiples of 2 to the n, for some reasonably large n?

tex

johnmsteele wrote:
>
> --- In ISO8601@yahoogroups.com, hjwoudenberg@a... wrote:
> > When I round trip a date and time to fraction of days, the round
> trip is
> > only approximate the same.
> >
> > hjw
>
> Broadly, two ways of handling:
>
> Others have touched on the idea of using seconds, (which Unix does)
> and there are variants:
> 1) Just use seconds as the fundamental interval. When days are
> needed, recover days as INT(SECS/86400) and seconds_within_day = MOD
> (SECS, 86400)
> 2) Use two words, one for days, one for seconds within days (may help
> get over precision issues)
> This would extend to any "tic" which is a submukltiple of a second,
> like milliseconds. (You don't say how fine a time interval you must
> resolve.
>
> The second option is to always round your answer. That requires
> knowing an acceptable resolution and carrying enough precision.
>
> If 1 second resolution suffices, there are 86400 s/day, and you have
> to carry AT LEAST 5 decimals in your day fraction. I would strongly
> recommend a guard digit, so use six digits. If you want milliseconds,
> nine digits in the fraction. Then when you go back to seconds, force
> round it to the resolution you have chosen.
>
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