In the labels.txt file the dates (Jan, Feb, Mar, etc...) should be listed. You can change these to whatever you wish. You can also change the number by using the Cheat Menu and working with the "interim turn". Also, you can change BC and AD in labels.txt.
Just make sure that you save a copy of the orginal labels.txt in your scenario folder and edit the copy so you don't corrupt any of the original files.