The offer ends when the clock strikes midnight on the 31st March. So I enter at 10.30 am on 31st March. Am I too late?

[cakeorwine]

Hear an advert on the radio. The offer expires when "the clock strikes midnight on 31st March".

So when do you think that is?

23:59 : 59 on 31st March is just a second before midnight. But is that midnight on the 1st April 00;00:00

So if I try to take advantage of the offer at 10:30 am on March 31st, have I missed it because it expired 10 and a half hours before

Or have I got plenty of time on that day to take advantage of it?

no you're not too late

It would make sense for a deal to.run til midnight on the last day of the month.

No, not too late. 00.00 is the end of 31st.

00.01 is the start of 1st

Doesn't 00:00 sound like the start of the 1st?

This is why 12 noon is often specified as a definite cut off re contracts etc.

23:59 is the end. 00:00 is the start,

We had 23:59 for the expiry of driving insurance.

I wonder what would happen if a crash happened at 23:59 : 31 seconds (so it rounds to 00:00

Well that’s past 23:59…

It's a very good point OP
I think it means end of 31st, and that most people would assume that.
But technically midnight on the 31st is the end of the 30th.

Just checked a real life example. DS applied for 6th Form with deadline on midnight 6 January. He submitted 20:45 6 January and got a confirmation of receipt.

This is the National Physical Laboratory

https://www.npl.co.uk/resources/q-a/is-midnight-12am-or-12pm

There is still disagreement and confusion about whether 00:00 is ‘today’ or ‘tomorrow’, but it is generally accepted that the moment the clock hits midnight marks the start of a new day. This means that 23:59:59 would be the last second of ‘today’, and the moment the clock hits 00:00, the new day has begun. This is why we celebrate the turning of the new year at the strike of midnight, and Cinderella was issued a strict warning about midnight in the popular fairy tale. It is common for transport timetables to use 23:59 or 00:01 to make it really clear which day they mean – same goes for deadlines on assignments and forms.

And that's true - when Big Ben strikes Midnight, it's the start of Jan 1st. At 00:00

I must check my current motor insurance.
Although I hope to have renewed it before the last day.
It always used to expire at 12 noon on a specified date.