Originally Posted by
c1223
Which means it's very unclear as to what you're actually getting as in my case the summary suggests 1 day (or at least so in my eyes). However I realise that maybe BA are counting 0001 on day 1 to 2359 on day 2, so are essentially charging 2 days of car hire.
No that's not quite how it works. It's all based on how Avis price it (which is the same as other car companies), which counts a 'day' as 24 hours from your time of pickup.
So in this case, because the BA system has set your pick-up at 09.00 (based on your flight time) and drop-off at 17.55 following day, the car hire counts as two days' rental because it's more than 24h from 09.00.
I agree with you that BA stating the dates but not the times on the summary page is very unclear/confusing in this respect, so you do indeed need to click through.
(As others have noted, for double TP purposes the maths is entirely different.)