Why Not Just Leave it at "Au Revoir"?

A while back I wrote about how you HAVE to say 'bonjour' and 'au revoir' when you go anywhere and meet anyone with a pulse. Otherwise you are just a rude uncouth foreigner, unversed in the sophisticated French ways. What I have since learned is that it is never just an 'au revoir' to qualify for a full farewell - the French add all sorts of greetings at the end, depending on the time of the day.

If you are saying goodbye anytime before 12, it should be 'bonne journee' OR 'bonne fin de matinee'.

If you are bidding adieu some time after 12, it is 'bon apres-midi'.

If it is past, say 4, it becomes 'bonne soiree', or 'bonne fin de la journee'.

It is just exhausting to keep up to return these said greetings properly. I always struggle, is it BON or BONNE - of course, it depends on the time of the day!

Also what gets me is, when I go pick up the kids around 4:30 and I say 'bonjour' to the moms like a good adaptable foreigner, some respond with a 'bonsoir' - so I'm still not sure what the cut-off time is when the 'jour' turns into 'soir'... why so complicated!?