function print_all(text)
for _, v in pairs(game.players) do v.print(text) end
end

script.on_init(function()
game.peaceful_mode = true
game.forces.enemy.kill_all_units()
end)

script.on_event(defines.events.on_tick, function(event)
if event.tick == 60 * 60 * 120 then print_all("warning1 ...

+++ - Supporting that!

Another Suggestion by myself is to add a command to activate/deactivate the peacful mode manually.