The ISO 8601-1:2019 hour range is 0 to 23

[Nethox]

gawk-5.1.1f/builtin.c:2127 contains:
        || (hour < 0 || hour > 23) /* FIXME ISO 8601 allows 24 ? */

The answer is no, according to the latest ISO 8601-1:2019 spec.

Section "Foreword":
> The main changes compared to ISO 8601:2004 are as follows:
> [...]
> — replacement of the term “midnight” with “beginning of day”, disallowing the 
> value “24” for hour;
https://www.iso.org/obp/ui/#iso:std:iso:8601:-1:ed-1:v1:en

Section "5.3.2 - Beginning of the day":
> For information interchange there is no representation of end of day. It is 
> recognized that the expression ‘24:00:00’ is used as a natural language 
> expression to denote end of a day; but for the benefit of clarity, ‘24’ shall 
> not be used to represent hour in accordance with this document.
https://github.com/dotnet/runtime/issues/12197#issuecomment-508299982

Regards.