Outbound PCR test question

[Gummibears]

Hello

Does anyone know (from the previous travelling to UK testing requirements) how strict the 3 day window for using your outbound test result on your return is?

Do you have until midnight on the 3rd day to use the result or is it a strict 72 hours? (e.g. if we have a test 11am outbound on the 21st would be have to travel back before 11am on the 24th or do we have until midnight on the 24th?).

I haven't seen much posted anywhere about this other than 1 paragraph on the government website.

Thanks in advance :-)