That’s what I mean by calendar units. These aren’t issues if you don’t try to apply durations to the “real” calendar.

(This is all in the context of cooldowns, where I’m not convinced the there’s any real ambiguity risk by allowing the user to specify a duration in day or hour units rather than seconds. In that context a day is exactly 24 hours, regardless of what your local savings time rules are.)

"exactly 24 hours" could still be anywhere between 86399 and 86401 seconds, depending on leap seconds. At least if by an hour you mean an interval of 60 minutes, because a minute that contains a leap second will have either 59 or 61 seconds.

You could specify that for the purposes of cooldowns you want "hour" to mean an interval of 3600 seconds. But that you have to specify that should illustrate how ambiguous the concept of an hour is. It's not a useless concept by any means and I far prefer to specify duration in hours and days, but you have to spend a sentence or two on defining which definition of hours and days you are using. Or you don't and just hope nobody cares enough about the exact cooldown duration