I'm trying to make a scenario for my friends and I to play; how do I change the year from say, 3000 BC to 2023 AD?

You have two options:

Create a GlobalDefinesAlt.xml file and change the "StartYear" define to 2023.
Or you could go into python, find the OnGameStart method, and add CyGame().setStartYear(2023) - both work just fine.

PS: BC dates have a dash (-) before them.