Is NOT, I repeat NOT the day of arrival and departure that counts - I dunno where that idea came from - It is the number of midnights spent in the UK. Read the summary of changes;
under the 183-day rule to count as a day any day upon which an individual is in the UK at the end of that day (i.e. at midnight)
So an overnight trip to the UK counts as one, a day trip counts as zero. Under the fallacy above, an overnight trip could have been 2, so it could have been worse.
I take issue with the UK's max 91 days average over 4 years rule - it is out of step with Europe, and half their maximum.