ISO 8601


A standard for timestamps.

It is roughly yyyy-mm-ddTHH:MM:SSZ, where

The offset from UTC is roughly written in this format:

Omitting the colon in the UTC offset

I tend to omit the colon in the UTC offset, but I'm not sure if this actually conforms to the standard.

Emacs does this by default:

For example, to produce full ISO 8601 format, use "%FT%T%z".

— Emacs docstring for format-time-string

(format-time-string "%FT%T%z") ; => 2021-06-07T20:21:18+0900

Shortened version

As far as I remember, there is also a shortened version defined in the standard, which removes all dashes and colons.