how do i make this to a negative (ShopRespawn < 180)?

void Update() {
shopRespawn -= Time.deltaTime;

	if(Shop.transform.childCount < 1 && shopRespawn < 0){
		
	}
	if(Shop.transform.childCount = 1 && shopRespawn < 180) {
		shopRespawn = shopStandard;
	}

so i have shop respawn to count down and then if there isnt a shop already in place (it deletes after every 3 minutes) instantiate (havent put that bit in yet) anyway i dont know how to make it so once shopRespawn has counted down it resets and doesnt keep resetting do i do it outside of the update function or is there another way?

id like to make it so this bit of the code is negative shopRespawn < 180

Like ((shopRespawn - 180) < 0) ??