Variation and declination

[penberth3]

Either. 24:00 allows us to refer unambiguously to a period of time on the same day. For example, the driver will be available to take you home until 24:00 on Tuesday. This makes clear we're talking about the end of Tuesday, not the start of it.

The UK will leave the EU at 24:00 (Continental time) on 29 March 2019. If we wrote 00:00 on that day, you'd be justified in thinking we would be independent of the EU at lunchtime on that day.

My car insurance expires at 23:59 on whatever date, which probably means 23:59:59 if you're bothered about the seconds.

 

[macd]

Um by that argument we would count down to midday so 10am would be 2 o’clock in the morning and 3am would be 9 o’clock in the morning...

No, by that argument, my tongue is at 9.00 o'clock. Depending on who the observer is, of course

 

[BelleSerene]

My car insurance expires at 23:59 on whatever date, which probably means 23:59:59 if you're bothered about the seconds.

We'd better not crash our cars in the minute between then and midnight when the next policy takes over, then!

 

[{151760}]

what time is midnight on a 24 hour clock? 00:00 or 24:00

Either. There's no confusion, they both indicate the same time, unlike 12:00, which could be either of two times.