I was taught that おはようございます when not used as a morning greeting, is basically saying 'hello, this is the first time seeing/meeting you today'. This is something that can be difficult to understand to anyone new to the language.
Let me give an example:
It's 10PM and a group of 'friends' enter a bar, and the bar tender, whom they are close to, welcomes them in and they all say おはよう to each other. This would be strictly translated as hello/hi/hey. Now understanding the greeting of おはよう/おはようございます as 'you are early', is more like, "Hello, I am seeing you for the first time today".
It can be temping as a speaker of English to take this phrase only as good morning. I have traveled to Japan and I remember I was walking down a busy street around 9PM to hear a man say おはようございます to someone. I was very confused at the time, but after learning more about the use of おはようございます, I have learned that it is very common in Japan to use おはようございます if someone is meeting each other for the first time that day.