A standard for timestamps.
It is roughly yyyy-mm-ddTHH:MM:SSZ, where
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yyyy is the 4-digit year
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mm is the 2-digit month
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dd is the 2-digit day
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HH is the hour
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MM is the minute
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SS is the second
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and Z is the offset from UTC.
The offset from UTC is roughly written in this format:
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Zstands for UTC. It's also referred to as Zulu time. -
Otherwise, it's either
+HH:MMor-HH:MM.
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+0900Shortened version
As far as I remember, there is also a shortened version defined in the standard, which removes all dashes and colons.
20210607T000000+0900