You probably have seen
ISO 8601 timestamps with fractional
seconds, such as this one:

% date --iso-8601=ns 
2023-04-20T18:45:11,094052607+02:00

However, many people don’t know ISO 8601 also allows for fractional
minutes and hours!

According to the standard, these timestamps are equivalent (rounded to a second):

2023-04-20T18:45:11
2023-04-20T18:45,18333
2023-04-20T18,75305

Note that in contrast to common scientific usage, the decimal part is
recommended to be separated by a comma and not a full stop, although
the latter is permitted too.

However, the standard does not specify the obvious next
generalization, that is, allowing fractional days. I thus propose to
extend ISO 8601 in the following way, which does not change the
meaning of valid existing representations:

The local time representation (after the optional time designator)
may consist of only a decimal fraction, which then is interpreted as a
multiple of 24 hours.

Thus, we can write the above timestamp also like this:

2023-04-20T,78137
2023-04-20,78137

Now, why would one want this? Essentially, there are three reasons:

First, it’s cute and an obvious extension of the existing format.

Second, it allows representing times of the
French Republican Calendar
in a natural way, which uses a decimal system as well: in this calendar,
the day is divided into 10 hours of 100 minutes and 100 seconds each.
Thus, the digits align directly to a decimal fraction of the whole day.
The above timestamp is then (computed using
fdate):

Primidi, 1 Floréal CCXXXI (231) 7:81:37

Note that we use local time here, not Paris time. If you insist on
using Paris solar time, you need to offset 9 ISO minutes and 21 ISO seconds,
which can be approximated as

2023-04-20T,77350+0009

Note that ISO 8601 does not allow for specifying offsets from UTC in
seconds (another obvious oversight).

Finally, the mechanism also supports the use of
Swatch Internet Time,
a late 90s decimal time system. Here, the day is divided into 1000 beats,
and the offset is fixed UTC+1 (for the Swatch headquarters in Biel):

2023-04-20T,739+0100

This is a bit more verbose than @739 but at least it’s an
international standard already!

NP: Tristan Brusch feat. Annett Louisan—Kein Problem

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