20091222

An introduction to decimal time

I just came across this page on the world-time-zones.org website.  It does a pretty decent job of describing decimal time.  I have a few comments about the actual decimal times displayed there, however.

The time in China shows hundredths of a day, since midnight Beijing standard time (GMT+8h).  It also displays decimal fractions of two digits, i.e. hundredths of hundredths, which AFAIK were never used by the Chinese, who had other ways of designating smaller units.  I also doubt they ever used Western digits to write the time this way, but that's OK for a web page written in English.

The so-called time in France is actually an hour (41.66 decimal minutes) fast.  The page must have been written in summertime, and the author forgot to account for DST.  France uses Central European Time, which is only one hour ahead of GMT/UTC in winter, but two in the summer.  Of course, during the Revolution, the French did not use CET, GMT or any standard time, but true solar time.  The mean solar time in Paris is 9 minutes 21 seconds ahead of GMT, or 6.49 decimal minutes.  But as I write this, their clocks are set one hour ahead of GMT, not two.

They use the modern colon separators, although traditionally the time is written in French by separating the hours and minutes with an 'h' and the minutes and seconds with an 'm', even today.  They also use '0' for the first hour after midnight, just as I do, although the revolutionaries used '10'.

Then they display 'UTC in Internet (Swatch) Time' and 'local time (in Internet Swatch time)', which makes absolutely no sense!  Internet Time is always one standard hour (41.66 .beats) ahead of UTC, and is never displayed relative to any other timezone.  And they show fractional .beats to 13 digits!  Swatch never even displays one digit, although some third-party applications might show two or three, but never 13.  And they did not include the obligatory '@' symbol.

And, although they mention that, 'Today, scientists, astronomers and computer programmers all persist in using decimal-based systems, using fractional days to give the precision required for certain recording and calculation', they fail to provide any examples, such as Julian Dates or Excel serial dates.

These are mostly minor quibbles, however.  Otherwise, they do a pretty good job describing decimal time.

MJD 55188.0

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