A standard for timestamps.
It is roughly
yyyy is the 4-digit year
mm is the 2-digit month
dd is the 2-digit day
HH is the hour
MM is the minute
SS is the second
and Z is the offset from UTC.
The offset from UTC is roughly written in this format:
Zstands for UTC. It's also referred to as Zulu time.
Otherwise, it's either
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 "%FT%T%z") ; => 2021-06-07T20:21:18+0900
As far as I remember, there is also a shortened version defined in the standard, which removes all dashes and colons.