It appears to be simply contrasting the meanings of "past tense" (which is a grammatical term) and "pastime" (which isn't). Without more context it is not clear why this is being done.
New words, one handy idiom, and a 2-minute quiz — delivered to your inbox to keep your streak alive.
Alexg00tA past tense is a verb form that refers to the past. A pastime has nothing to do with grammar; it’s a hobby or activity."It may just be punning: 'pastime' vs 'past time'. Or perhaps a facetious correction of a correspondent's spelling mistake.