We revisit multi-slit diffraction with a scaling of space and time to exploit an equivalence of wave packets describing quantum free and harmonic oscillator (HO) motion. We introduce a co-moving frame of space and time coordinates to define an effective, time-independent, HO potential that confines and directs initially displaced wave packets along the classical phase space of the oscillator. $N$-slit diffraction in the lab frame from the slits to a distant detector and the familiar spreading of the propagating wave front is then described as the propagation of HO wave packets over just a quarter cycle of the oscillator and confined to the HO potential well. This connection with the co-moving frame suggests a simplified time of flight experiment wherein the arrival time distribution of diffracted particles hitting a \emph{single point detector} in the lab frame images in the co-moving frame as the \emph{full} diffraction pattern from grating to detector.