How to print ISO8601 date using 'date' on command line

Use one of these commands to print the ISO8601 time using the date tool included in most Linux distributions.

date -I # 2019-04-06
date -Iseconds # 2019-04-06T17:22:49+02:00
date -Ins # 2019-04-06T17:23:08,505995625+02:00

For reference see the date manpage.