Zip and fly. You can look them up in a dictionary. 

Because time = it, you need to conjugate the verbs into "zips" and "flies".

I think it is an expression..

For example: "Time fly by".... i think it is about something that happened. Is an expression for the past... 

"Time zip by" for zip, i think in zipper, so I guess if the zipper goes up..... Up. So it is just and expression for the future.. Something is going to happen.. 

and is just my opinion...