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#### Question Detail

The date is approaching (in fact, 2022-01-08), when Unix time turns 19000-days old. Currently, there is a way (namely, `%s`) to display `Unix second` in dates, but there seem to be no way to display `Unix day` in date formatting. How does one display `Unix day` in dates? If not possible, then how to introduce new date formatting symbol (`%?`) symbol for it?

Why?

1. Epoch date is easy to compute just dividing `Unix seconds / 86400`, and works well as natural decimal time, where:

1 dYear = `1000 days`, 1 dMonth = `100 days`, 1 dWeek = `10 days`,
1 dHour = `0.1 days`, 1 dMinute = `0.001 days`, 1 dSecond = `0.00001 days`.

2. `Unix day` digits, unlike `Unix second`‘s digits, approximate lengths of ordinary concepts of time with less than 1 order of magnitude, thus work well as `decimal time and date`, where:

For example, now is `Unix day = 18993.94830052069` means:

``````18993.94830052069
\ \\\ \ \ 30.052069 th decimal second
\ \\\ \ 48 th decimal minute
\ \\\  9 th decimal hour
\ \\ 3 rd day of week
\ \ 9 th decimal week
\  9 th decimal month
18 th decimal year
``````

`Unix day`, interpreted this way, has the benefit of convenience of use as natural support for decimal time, so some people use it and need it.

Ultimately, I’d like to know a path for incorporating this display of time as one of the supported ways of displaying date and time in *nix systems.

date +%s displays the Unix second, is there a way to display Unix day with it?

No, it is not possible.

how to introduce new date formatting symbol (%?) symbol for it?

Download the sources of the `date` utility implementation that you are using and patch it with a custom modifier. GNU date (if you are using GNU date…) is part of GNU coretuils, see https://www.gnu.org/software/coreutils/ . Date utility follows POSIX specification.

I do not have ever encountered any need for a “number of days since epoch with fractional part” format. I do not believe it has any practical usage. I do not believe such a formatting specifier should be added to `date`. I see no value in it over “number of seconds since epoch with fractional part” (`date +%s.%N`), it’s just a change of unit. I wouldn’t want those hard-working GNU developers that are giving their work to the world for free, for them to spend time on something that has no practical application. I would like them to they spent their time with their families or on other more important things. As such, I encourage you to not take developers time, unless you can strongly prove that the community requires such a formatting specifier for practical reasons.

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