Agreed - in my mind the easiest way to consider it is to consider the 29th as an infinitesimally short day on non-leap years. On the 1st of March that day has p

Agreed - in my mind the easiest way to consider it is to consider the 29th as an infinitesimally short day on non-leap years. On the 1st of March that day has passed and they are 18, on the 28th of Feb that day hasn't started. The same tactic works for working out when one month from the 31st August is