The midnight (end of day) 24:00 notation is a curious case. The copy I have of a draft of ISO 8601:2019 removed it, which I thought was a shame. But the end of this comment https://old.reddit.com/r/ISO8601/comments/mikuj1/i_bought_is... suggests that this change didn’t survive into the final standard, so 24:00 is still allowed, as in older editions of ISO 8601.
This relates to how I learnt about ISO8601 as a teenager (c. 2005)
My parents had an oven manufactured by Neff (part of the BSH group including Bosch and Siemens). This oven displayed midnight as 24:00. I was rarely in the kitchen at midnight, so it was years until I just happened to walk into the kitchen at the right time to 24:00 on the display. It was so weird I think I continued to stare at it for the remainder of the minute until it ticked over to 0:01.
I wrote directly to Neff to ask them about it. I speculated that it was a design decision to reduce ambiguity between the clock and the timer countdown, which shared the same display, and asked them if this was the case.
I can't remember what email address I used, but after a bit of bouncing around and piquing people's curiosity it ended up with someone in their engineering department. They looked into it and actually quoted ISO8601 to me. They said, while they agree it is a bit strange, it's not actually out of spec and therefore not technically a bug. They didn't agree with my theory about ambiguity since many other times would still be ambiguous (e.g. 0:01). But since the clock circuit is a sourced component they could only speculate on why 24:00 was chosen over 0:00. They also confirmed that Bosch and Siemens ovens both display 0:00 and only Neff displays 24:00.
I've since noticed that 24:00 is used on train timetables when the time of arrival is midnight.
