The easiest way to remember this is that BC stand for Before Christ and AD stand for After Christ. In reality it does not but it is the easiest to remember. So if you say the Egyptians were present 1500 years before Christ and the Aztecs were present 1500 years after Christ that means that the Egyptians were here at least 3000 years before the Aztecs. In reality we do not know when the Egyptians came to be as they were already an advanced civilization when they made the earliest artifacts we have from them from about 5000 BC. And there are signs that they were conducting archeological digs back then and reusing what was even then ancient works.