Just open up the folowing folders

C:/ Programs\Total War\Mediavel - Total War\Campmap\Startpos\

Then from there choose what time period you want to play ( MAKE A COPY OF THE FILE BEFORE YOU MAKE ANY CHANGES JUST IN CASE )

Choose Early, High, Late and open the text file.START DATE.

Change
SetStartDate:- (xxxx)
to what ever you want

And set the
SetLatePeriodStartDate:- (xxxx) to the same date.
If you choose to play in the Early it would be SetEarlyPeroidStartDate:-(xxxx)
and High would be SetHighPreiodStartDate:-(xxxx)

You must use 4 Numbers ie: 1000, 0050 or what ever you want.