This would be set before a match is started in the same place the factions/players, etc are chosen.
It could be set in minutes either from a list (1, 5, 10, 20, 30) or just entered customary.
Peace time starts when the game does and it prevents battle units/buildings from being built or produced.
So that battle units can be differentiated from other units a flag or something would help, this way rather than disabling the units I could set it so peace time prevents buildings that create battle units from being built during peace time.
Idea from Stronghold, always loved peace time as it allowed for better planning and longer matches.