Aggretsuko - Trying to be cool in public

Oh, I get it, she is literally a secretary bird.

I think the choice to go with "calendar" instead of "part timer" was to make it clearer that Ton is indicating Retsuko's "days are numbered", because in America "part timer" is a phrase used to refer to people who only work a job part time (like a fast food employee who only works the lunch shift).