Hi Doc. You need to go into the 'Mods' folder and open either the DH Full or DH Lite folder, depending on which you're playing. Once inside that, go into the 'scenarios' folder and select the EUG file for either the 1914 or 1936 scenarios. Open that and use the 'Find' function to search for 'enddate'. The third, and last, find you get will be the end date for that scenario and you can edit that to whatever you want.
Edit: Whoops, left in the wake of the AC once again. It is 'enddate' though, rather than 'end_date'.