That's not how they work. Between timer 0 and timer 17 it is going at 1.7x speed, which takes 10 frames. The hitboxes are after the 17 timer and so is the 0.8x. From 17 until the end of the move it is at 0.8x speed (this sets the speed, not stacks with current speed).
Basically:
0, 1.7, 3.4, 5.1, 6.8, 8.5, 10.2, 11.9, 13.6, 15.3, 17, 17.8, 18.6, 19.4, etc
The 17 timer is reached in 10 frames, so it hits on frame 11 of that animation. Smashes have multiple parts though (pre-charge, charge, post-charge), and the startup is 1 frame long, making it hit on 12 overall.