`date` command on OS X doesn't have ISO 8601 `-I` option?

Question

In a Bash script, I want to print the current datetime in ISO 8601 format (preferably UTC), and it seems that this should be as simple as date -I:

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Answer 1

In GNU date date -I is the same as date +%F, and -Iseconds and -Iminutes also include time with UTC offset.

$ date +%F # -I or +%Y-%m-%d
2013-05-03
$ date +%FT%T%z # -Iseconds or +%Y-%m-%dT%H:%M:%S%z
2013-05-03T15:59:24+0300
$ date +%FT%H:%M # -Iminutes or +%Y-%m-%dT%H:%M%z
2013-05-03T15:59+0300

-u is like TZ=UTC. +00:00 can be replaced with Z.

$ date -u +%FT%TZ
2013-05-03T12:59:24Z

These are also valid ISO 8601 date or time formats:

20130503T15 (%Y%m%dT%M)
2013-05 (%Y%m)
2013-W18 (%Y-W%V)
2013-W18-5 (%Y-W%V-%u)
2013W185 (%YW%V%u)
2013-123 (%Y-%j, ordinal date)
2013 (%Y)
1559 (%H%M)
15 (%H)
15:59:24+03 (UTC offset doesn't have to include minutes)

These are not:

2013-05-03 15:59 (T is required in the extended format)
201305 (it could be confused with the YYMMDD format)
20130503T15:59 (basic and exteded formats can't be mixed)

Answer 2

The coreutils package provides GNU versions of tools. To install:

$ brew install coreutils

You can see what's provided:

$ brew list coreutils

Notice it comes with date:

$ brew list coreutils | grep date

This is the standard GNU date command so it'll take the -I switch:

$ gdate -I
2016-08-09

Answer 3

You could use

date "+%Y-%m-%d"

Or for a fully ISO-8601 compliant date, use one of the following formats:

date -u +"%Y-%m-%dT%H:%M:%SZ"

Output:

2011-08-27T23:22:37Z

or

date +%Y-%m-%dT%H:%M:%S%z

Output:

2011-08-27T15:22:37-0800