This mod is exactly what I was looking for as I much prefer to turtle my way to endgame and then face the baddies.

That said, I made a mistake of setting the timer to a value I didn't like and had a bunch of savegames with the original timer. After a bunch of trial-and-error of attempting to sync startup changes to the saved games I had, I could never get a different truce timer value to take effect (the savegame startup value for Truce Mod seemed to always overwrite my settings in the main menu).

I followed these steps to reset the timer:
1) Uninstall Truce Mod
2) Load save game and when Factorio prompts you to sync mod settings, load the save without Truce mod enabled
3) Reinstall Truce Mod and make sure the correct value is set for the countdown timer in the main menu mod settings
4) Re-load save game and you should see the new timer

I'm sure if you used a save game editor you might be able to change this but for those that want a native-UI workaround, here you go. :)