How do I make enemy obstacles progressively faster?

I'm trying to make enemies come at the player faster as time moves on. Right now I've got something like this:

System: Every 5 seconds - Create object along with a bullet behavior attached to the object. I want to make it so the timer decreases every 15-20 seconds.

Thanks

I basically want to increase the bullet speed of an object every 15 seconds

just do every 15 seconds, set enemy bullet speed to current bullet speed +10 or 20 or 50

or you can do a is on screen set bullet speed, to self.bullet.speed +1 that will make enemys speed up as long as they are on screen, if its a 2d spaceshooter will be good if its a platform, it wont be that good

Thanks! that was the perfect formula I needed.