`date` command doesn't have an option for ISO-8601 week format input?

Question

I occasionally use the date command to check the current or a future date's week number based on the ISO-8601 week-date calendar. To do this I simply use: date +%V to check the current ISO week number and date -d "2021-12-25" +%V to check the ISO week number of another date (Christmas day in this case).

However, it seems like it is not possible to do this in reverse. That is, get the date in "month-date" format given a date in "week number" format. For example, if I run date -d "2021-W50-3", it generates an error saying the format is not supported. Why is it not and is there any way to input or store dates using this format?

Answer 1

GNU date, with that non-standard -d option, only supports a limited set of date-time formats documented at info date 'Date input formats' (and online here).

For a date implementation that supports more date formats including the one you're referring to, you can use the date implementation from ast-open (which can also take an input date with -d like GNU date).

That can also be the date builtin of the ksh93 shell if built as part of ast-open (rarely the case though), or instead of date, you can use its printf builtin (which is always included) and its %T specifier (which bash copied though without the time parsing part):

$ builtin date
$ date -d '2021-W50-3' +%F
2021-12-15
$ printf '%(%F)T\n' 2021-W50-3
2021-12-15

From the bash shell (or any other shell), you can always do it as:

$ ksh93 -c '"${@:0}"' printf '%(%F)T\n' 2021-W50-3
2021-12-15

though note that depending on the system, ksh93 may be known as ksh, ksh2020 (or even sh like on Solaris 11+; though it's also known as ksh there), or not at all, as ksh93, like bash or zsh is not installed by default on all systems.

Note that while standard strftime() can produce dates in that standard format with the %G-%V-%u specification, the corresponding standard function to parse timestamps (strptime()) doesn't support %G nor %V (nor does standard UNIX getdate()).

GNU strptime() is currently documented to understand them (as an extension over the standard) but ignore them (see info libc strptime). FreeBSD strptime() is documented to support the same directives as strftime() but in my test on FreeBSD 12.2 (using zsh's strftime -r builtin), it doesn't seem to work there.

POSIX, GNU and freebsd strptime() do support %W though.

• zsh:

  $ strftime -rs t %Y-W%W-%u 2021-W50-3 && strftime %F $t
  2021-12-15
  

• busybox:

  $ busybox date -D %Y-W%W-%u -d 2021-W50-3 +%F
  2021-12-15
  

%Y-%W is not the same as %G-%V though, it just happens to coincide in this case. %W on the first of January is 00 unless that's a Monday in which case it's 01. Whilst %V (the ISO week number) is either the first week of the year (01) or the last week of the previous year (52 or 53) depending on whether that week has more days in the previous or current year.

Answer 2

It is possible to get the precise ISO week date (including the flip into previous or next year) by using two executions of date: one to calibrate a known day (I chose 01-June), and one to adjust by the number of days for the ISO week/day difference. This is unaffected by leap years -- that's the whole point of using ISO weeks instead of months.

This is a function that converts 2021-W50-3 format to 2021-12-15, with tests that show the year roll-over.

#! /bin/bash

isoDate () {

    declare -a dR   #.. Required date
    declare -a dC   #.. Calibration date.
    printf 1>&2 '\nAsked %s\n' "${1}"   #debug
    IFS='-' read -a dR <<<"${1}"
    IFS='-' read -a dC <<<"$( date -d "${dR[0]}-06-01" '+%G-W%V-%u' )"
    dR[1]=${dR[1]#W}; dR[1]=${dR[1]#0}
    dC[1]=${dC[1]#W}; dC[1]=${dC[1]#0}
    declare Adj=$(( 7 * (${dR[1]} - ${dC[1]}) + ${dR[2]} - ${dC[2]} ))
    date -d "${dR[0]}-06-01 + ${Adj} days" '+%Y-%m-%d'
}

    Ymd=$( isoDate '2021-W50-3' )
    printf 'Found %s\n' "${Ymd}"
    date -d "${Ymd}" '+Check %G-W%V-%u'

    Ymd=$( isoDate '2020-W01-1' )
    printf 'Found %s\n' "${Ymd}"
    date -d "${Ymd}" '+Check %G-W%V-%u'

    Ymd=$( isoDate '2011-W52-7' )
    printf 'Found %s\n' "${Ymd}"
    date -d "${Ymd}" '+Check %G-W%V-%u'

And the results:

$ ./isoDate

Asked 2021-W50-3
Found 2021-12-15
Check 2021-W50-3

Asked 2020-W01-1
Found 2019-12-30
Check 2020-W01-1

Asked 2011-W52-7
Found 2012-01-01
Check 2011-W52-7
$