'Good evening' is good any time after it starts getting dark. If it's earlier, use 'good afternoon'. Both of these also work for 'good bye' at the same appropriate times (though they are a bit formal). 'Good morning' works any time after about 3 a.m., depending somewhat on who has been up all night and who's been sleeping. 'Good night' and 'good bye' only work when you are leaving.
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