A friend of mine told me that you can say "good morning" to a person at any time, if you are seeing him/her for the first time on that day. Is this true?
Perhaps that is a regional thing; where I am from in Canada, we laugh at each other good-naturedly if we say "good morning" after 12:00 noon, as if we slept in late.
If it is approaching 12:00 noon, we will look at our watches first to make sure we shouldn't say "good afternoon" instead.
Likewise, we will say "good evening" as a greeting after around 6:00 p.m., and "good night" when parting.