Ok, I'm not sure whether this is what you're asking, but in \Data\world\maps\campaign\imperial_campaign\descr_strat.txt you can edit the start and the end date. The lines are just below the first ones where you edit to play all factions. The rest of the buildings you would have to edit manually, of course, further down in the file for each region. Just follow the way the code is implemented, with { and } and building descriptions as per export_descr_buildings.