Time and motion are closely related. This is because motion takes place over time. Illustrating motion on a 2-dimensional surface can be complex. Yet, it is a necessary skill illustrators must work to perfect.
There are many ways for one to illustrate motion and the passing of time. The most common technique is to add jet stream like lines trailing off from the object or subject that is in motion. Take this comic book cover as an example. We all know the flash is incredibly fast. In order to illustrate this point the illustrator had one solid jet stream trailing from the flash to indicate he is moving extremely fast.
Also, take note of how the two subjects appear to be growing larger in comparison to their jet streams. This indicate another way to represent motion and time; scale. Scale can be reduced or expanded upon to convey the passing of time and of motion.
Although you cannot see it on the cover above, another way to induce motion in comics is to play off of the reader’s imagination. The gutter between the panels allows the reader to imagine the motion or steps that connect panels.