Those two LoC links are for the 2016 draft of the 2019 specification. Hard to say how much it varies. And the ISO link only lets you see the definition of ter

Those two LoC links are for the 2016 draft of the 2019 specification. Hard to say how much it varies. And the ISO link only lets you see the definition of terms and symbols. Actual syntax details are all locked up. https://www.loc.gov/standards/datetime/ https://www.loc.gov/standards/datetime/ seems to be a usable summary of the 2019 spec, though being just a summary means it probably excludes discussion of finer details and considerations which might be important at times.