Sure, it could be the planet of "The Little Prince" :)
Or it could be midnight, 12 o'clock sharp, and you want to say both (just to be covered). Or it could be past midnight when you should say good morning, but you want to say goodnight too, to show that you're going to sleep. Or someone asks you to translate "bonjour" and "bonne nuit". Or... or... or... well, the possibilities are endless!
Yes, I guess after 8 would be good for καλοβράδυ. Greeks are not particularly precise and will understand if you use an expression a bit outside the usual time. So, you wouldn't normally say "' καλημέρα" at 4pm...but if you did you'd just get a smile.
"Καληνύχτα" in the morning well, someone might say..."I think you mean "καλήμερα". But no one will be offended.
Καλημέρα is litterally good day and not good morning. We already know we won't use it in the evening or the night, but can we use it in the afternoon, so that it could be translated by good morning or good afternoon as well? It would be very similar to the French bonjour/bonsoir. Moreover, bonne nuit (good night) can't be used as a greeting, just like in Greek.
Kαλημέρα literally means just good morning. For hello there is γεια/γεια σου/γεια σας, and for good afternoon there is καλησπέρα. I do understand that hello might be used instead of good day in English (although I see it more like a simple greeting rather than a "wish"), but not all translations match the Greek word. Καλημέρα is used for a specific reason is Greek, at a specific time of the day. :/
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