Please, oh Please, do not do this.
In general, the output of date is controlled by
$ LC_TIME=ja_JP.utf8 date; date'
2017年 7月 13日 木曜日 18:00:22 EDT
Thu Jul 13 18:00:22 EDT 2017
And it may be very (and I mean: really very) difficult to parse a date string even if it seems natural and obvious to the reader.
Like the Japan format above.
It could not be used back as input to the date command.
You can edit the locale file to fit your needs and even make it the default.
First, a quote:
Our units of temporal measurement, from seconds on up to months,
are so complicated, asymmetrical and disjunctive so as to make
coherent mental reckoning in time all but impossible. Indeed, had
some tyrannical god contrived to enslave our minds to time, to make
it all but impossible for us to escape subjection to sodden
routines and unpleasant surprises, he could hardly have done better
than handing down our present system. It is like a set of
trapezoidal building blocks, with no vertical or horizontal
surfaces, like a language in which the simplest thought demands
ornate constructions, useless particles and lengthy
circumlocutions. Unlike the more successful patterns of language
and science, which enable us to face experience boldly or at least
level-headedly, our system of temporal calculation silently and
persistently encourages our terror of time.
... It is as though architects had to measure length in feet, width
in meters and height in ells; as though basic instruction manuals
demanded a knowledge of five different languages. It is no wonder
then that we often look into our own immediate past or future, last
Tuesday or a week from Sunday, with feelings of helpless confusion.
—Robert Grudin, ‘Time and the Art of Living’.
Because it mostly conforms to the ISO-8601 date format.
Which could be printed as this:
$ date -Id -d 20170713
Yes, you can use such an awkward format (with busybox date or BSD date):
$ busybox date -D '%Y%d%m' -d "20171307"
Thu Jul 13 00:00:00 EDT 2017
And even print it in the same format:
$ busybox date -D '%Y%d%m' -d "20171307" +'%Y%d%m'
Yes, you can provide a format, and above in
2 is an example.