I have implimented a 'turns per era' setting into my mod that works by increasing the cost of techs for the 'next' era based on the current turn. The main purpose is to prolong the lifespan of various units and equalize the amount of game time in each era without locking everybody into the same tech level. Even so, the change does a good job of maintaining a sense of historical accuracy with the dates, within reason, perhaps even conceptually
For example, my axemen and swordsmen roam the world until about 600-800 AD when they're replaced with macemen and pikemen who are the mainstay of my army until muskets hit widespread use around 1400-1500 AD followed by riflemen shortly thereafter and finally infantry and tanks in the early 20th century. Each respective era gives me roughly 67, 100, 150 or 300 turns of gameplay depending on the speed setting I chose.
So it's definately possible to mod the game speeds to even out the game time in each era, but it's more than a simlpe XML change as those never seem to work out just right. Any changes in the XML file will have a general effect over the game as a whole but it's very difficult to control the effects with any degree of accuracy. Even going through individual technologies and adjusting their costs only works some of the times, other times it exagerates the tech lead you'll already have by then.
In the default game speeds they increase the number of turns and the costs of buildings, units, wonders and technologies so it evens out. You can go in and modify them yourself, as long as you increase the number of turns to compensate for any increases in tech costs you should be fine, getting those turns to match up with dates is another thing all together since the game uses 'time slices' where each turn represents 'x' number of months for 'y' turns, then you enter the next time slice in which each turn represents a different number of months for a differnt number of turns, and so on.
Fun stuff
