The date command is crufty.

It can be used for both setting and displaying the current date.

Setting the date requires root privilege and is obsolete since modern systems have a daemon. On Linux the daemon is ntpd, though I think systemd has taken over the responsibility. On Mac the daemon is timed.

The date flags are not standardized for the most part. POSIX only specifies two flags: -u and +CONVERSION_SPECIFICATIONS. As you can see from the table below, flags available on Linux are not necessarily available on Mac and vice versa.

The %s conversion specification for the UNIX epoch is not required by POSIX but is supported on both Linux and Mac.

Linux Mac
set current date $ date MMDDhhmm[[CC]YY] $ date [[[mm]dd]HH]MM[[cc]yy]
set current date using 8601 timestamp $ date -s 2018-10-01T00:00:00 $ date -f %Y-%m-%dT%H:%M:%S 2018-10-01T00:00:00
set current date using UNIX epoch $ date -s @1543336412 $ date -f %s 1543336412
set current date using input format $ date -f %Y%m%d%H%M%S 20181001000000
display current date $ date
Tue Nov 27 08:19:29 PST 2018
$ date
Tue Nov 27 08:19:29 PST 2018
display current UTC date $ date -u
Tue Nov 27 16:27:30 UTC 2018
$ date -u
Tue Nov 27 16:27:30 UTC 2018
display current date according to locale $ LC_ALL=fr_FR date
Mar 27 nov 2018 08:19:24 PST

$ LC_TIME=fr_FR date
Mar 27 nov 2018 08:38:11 PST
display current date using output format $ date +%Y-%m-%dT%H:%M:%S
2018-11-27T08:29:46
$ date +%Y-%m-%dT%H:%M:%S
2018-11-27T08:29:46
## mediumblue|display current date as UNIX epoch## $ date +%s
1543336412
$ date +%s
1543336412
parse date using input format and display $ date -j -f %Y%m%d%H%M%S 20181001000000
Mon Oct 1 00:00:00 PDT 2018
parse ISO 8601 timestamp and display $ date -d 2018-10-01T00:00:00
Mon Oct 1 00:00:00 PDT 2018
$ date -j -f %Y-%m-%dT%H:%M:%S 2018-10-01T00:00:00
Mon Oct 1 00:00:00 PDT 2018
parse UNIX epoch and display $ date -d @1543336412
Tue Nov 27 08:33:32 PST 2018
$ date -j -f %s 1543336412
Tue Nov 27 08:33:32 PST 2018
parse date using input format and display using output format $ date -j -f %Y%m%d%H%M%S 20181001000000 +%Y-%m-%dT%H:%M:%S
2018-10-01T00:00:00
display one hour before current date $ date -d '-1 hour' $ date -j -f %s $(($(date +%s) - 3600))