Calendar: Why do hours, minutes and seconds go to 24/60/61 and not 23/59/60?

[Ekleog]

I'm trying to consume a DateTime, and turn it back into a number of seconds since posix epoch.

I'm having questions related to the range of IsoHour & co.

The ranges are currently documented as being [0, 24], [0, 60] and [0, 61]. I do not understand why they contain the last value?

It looks to me like in early 2021, the units were set with an exclusive bound at the end, seeing how "week day" is 7 and one of the rare things everyone seems to agree on is the duration of a week.

However, the comment about the current allowable ranges seems to have been introduced without explanation.

Do you know why the allowable ranges are 0-24, 0-60, 0-61, and not 0-23, 0-59, 0-60 (or 1-24, 1-60, 1-61)? Are there some calendars that mean different things by the hours / minutes / seconds than ISO 8601, and if so why is this type called IsoMinutes? (though some versions of ISO 8601 seem to allow 24:00:00, that wouldn't explain for 60 minutes / 61 seconds)

[sffc]

The extra second is to handle leap seconds, and then all the fields support one greater than the maximum cyclic value due, I believe, to accommodate certain situations that express the difference between "last unit of previous time period" versus "first unit of next time period".

[Ekleog]

Got it, thank you! (And sorry for not having used discussions, I've literally never been on a project that actually uses them yet 😅 )