We use good day in serbia from like 9 to 5 too(6 in the summer or even later as it is still bright outside),good morning to 9 should be optimal,maybe even 10,and evening until 5-6 till the rest of the day. Good day slips its way no matter the time,force of habit. Anyways back to italian,I feel like this should have been translated as more of a hello,if not a good day. If it's more of a ''universal'' greeting,or a multitool,i still feel like it would be easier to pick up on that later on,rather than translating it as good morning,when the word in italian obviously means something day. Sorry for the lenght
Yes, ClassiDuo is correct. "Buona giornata" is used to wish a person a good day; it is not used as a greeting. For formal greetings, you can use "buongiorno" from morning until the sun starts to set, then use "buona sera." "Buon pomeriggio" is possible but not really used. "Ciao" is fine at any time with people you call by their first names (informal), and "Salve" is also fine all day long, and less formal than "buongiorno." "Buonanotte" is used when leaving people in the evening, or going to bed (just like "good night" is used in English).
I spent 15 days in Italy and went to different cities like Milan, Venice and Rome and never heard once anyone say buonmattina. They either say buongiorno or buonasera. And they keep saying buongiorno till very late afternoon, so I figured it meant more like good day rather than good morning.
Literally translated that is precisely what it "means", however since they use it in the mornings, they tend to ONLY accept "Good Morning". Personally, I think "Good Day" should be accepted since that is also often used in the mornings in many/most English speaking countries. However, they're being picky on it and you MUST say "Good Morning." Just FYI
DL doesn't accept "Good day" as a translation for "Buongiorno?" Only "Good morning?" How about ¨Good afternoon?" I agree "Good day" is antiquated in English, but since it's the literal translation it should be accepted. All three of these variants should be accepted! If they aren't, report them! It's important for the reverse tree too (for Italians who are learning English).
We do say "Good morning." which this is also used for. People are falling into the trap of translating word for word again. "Buongiorno" (Literally good day) is used for "Good morning" in English.
For Nata_Vi below:
It is also "Good day", that is just not as used in modern English than in olden times. It is funny that in English "Good morning" or "Good afternoon" is preferred to "Good day" and in Italian "Good day" is preferred to "Good morning" or "Good afternoon" for greetings. Buonasera means "Good evening" , but it is also used in the late afternoon. I think that is why Duolingo does not accept "Good afternoon" for "Buongiorno". They probably already used "Good afternoon" as an alternate translation for "Buonasera".
How old was your Italian tutor? Was he or she born in Italy? It is not considered "wrong" to write it as two words, as it was common in the 19th century and before. But in modern times I have yet to see a single example of a living author (or any native speaker) write it like that. Here is an article: https://learnitalianwithlucrezia.blog/2016/09/18/buon-giorno-o-buongiorno/
and a forum discussion from wordreference.com: https://forum.wordreference.com/threads/buongiorno-vs-buon-giorno.1527975/
And some stats, showing that during the 20th century the frequency of writing it as one word grew: https://books.google.com/ngrams/graph?content=buongiorno&year_start=1800&year_end=2008&corpus=22&smoothing=3&share=&direct_url=t1%3B%2Cbuongiorno%3B%2Cc0#t1%3B%2Cbuongiorno%3B%2Cc0