Trying to get a spawning actor to wait X seconds before spawning other actors

Hey gang,
trying to make a shmup that progressively gets more difficult by basically throwing more and more enemies at you over time. I have 3 spawn actors. the first two spew out weak enemies, and I'd like the third one to spawn a tougher enemy, AFTER 15 seconds has passed.
It's tricky for me because I've got all my "screens" (title, main play area, game over, etc) on the same "scene", so the game.time is different with each play session. (depending on how long the player waits before pressing start)

i've got a boolean 'gameStart' attribute that is activated when the player presses start and begins to play, so is there a way to create a rule that tells the Spawning actor to wait 15 seconds after 'gameStart' is set to True to begin spawning the tougher enemy?

thanks for any help!