Entering Boss Portal in Co-op

In co-op, do you need to enter the boss portal a couple seconds before the time is up? When one teammate enters the portal, there is a delay before it teleports everyone to it. I think it has triggered overtime before when there were ~5 seconds left and one teammate entered the portal but maybe it didn't? So now my friends enter the portal with at least 10 seconds left to make sure it doesn't trigger overtime and we die from a buffed boss. I just wanna make sure this is how that works, as sometimes those 10 seconds can be used for buying stuff.