Hi folks can I just double check the requirements for my trip?

I have an Edinburgh to NYC trip:
Day one: Edinburgh to Heathrow, then a long overnight, over 12 but under 24 hours.
Day two: Afternoon flight to NYC.

Does the one day rule kick in:
a) the day before the first flight on my itinerary.
b) 24 hrs before the first flight on my itinerary.
c) the day before my flight to the US.
d) 24 hrs before my flight to the US.

I’ve read a lot (not all) the posts here and my mind reads things differently every time!

Thanks for your help in advance.