Hi. On the 5:2 your fast day really lasts 36 hours. So for instance last meal on a Monday at 6pm. Fast day Tuesday 500 cals, breakfast on Wednesday 6pm. So really you are consuming 500 calories in 36 hours. This is what the 5:2 book says anyway.

Yes I count my fast day starting straight after the clock clicks over to the new day the second after midnight. But because my last meal was maybe 5 hours before that, I have already got 5 fasting hours under my belt. Then I have my day of minimum calories and another nights sleep, and wake up feeling very pleased with myself and looking forward to breaking my fast! My fast day ends at the stroke of midnight, but because I wait for breakfast time to eat, the whole length is about 36 hours.