You can change the starting Era before beginning each game.
In order to mod the starting date you must go to the following file
XML\GameInfo\Civ4GameSpeedInfo
The game starts in 4000BC but in this file you determine the time intervals between turns.
Remember, Classical starts 10% through the timeline, Medieval starts 20% through the timeline, all the way to Future, which starts 60% through the timeline.
I don't know exactly what you are shooting for but with some Math you should be able to have starting sates correspond to the intended starting era.
I'll have to look up how to change the starting date from 4000BC.
EDIT: Found It! in the file XML\GlobalDefines there is a constant that determines the start year
<Define>
<DefineName>START_YEAR</DefineName>
<iDefineIntVal>-4000</iDefineIntVal>
</Define>
The negative indicates BC. You should now be able to alter time however you want.