The Linux date command can be used to get the current date (and also to modify the date of your machine, but I've never used it for that). It's useful if you want to programmatically add a date to a filename, either one time or as part of a regularly-scheduled or repeated task.

Here are some of the frequently used formats I use:

Command	Sample Output	Notes
date +%F	2021-03-19	If you just need the date
date +"%F %R"	2021-03-19 22:43	Like the date and time of this post
date +"%Y-%m-%dT%H:%M:%S%z"	2021-03-19T22:43:52-0400	ISO 8601 time in the local time zone.
date -Iseconds	2021-03-19T22:43:52-04:00	Same, except including a ":" in the offset (does not work on macOS)
date -u +"%Y-%m-%dT%H:%M:%S%z"	2021-03-20T02:43:52+0000	ISO 8601 date in UTC
date -u +"%Y-%m-%dT%H:%M:%SZ"	2021-03-20T02:43:52Z	Same, but using Z instead of +0000
date -u -Iseconds	2021-03-20T02:51:58+00:00	Same with colon in the output (does not work on macOS)

For more information, see this Stack Exchange post or the date man page (along with the strftime man page for formatting options).