Well both have other meanings but when combined with time they are pretty similar. A differentiation would be that spend time tends to suggest that is more of active time. being busy doing something. Pass can be more passive. i.e. i am waiting for something to happen. there isn't always a definite difference. I will spend time with my family next weekend. i will work to pass the time until i see them.

"Spend time” is used to talk about the time you do an activity. "Pass the time" means doing something to make the time pass faster while you are waiting for something else. "I'm spending time at my mother's house." "I spend my time reading books." "I'm spending two weeks in London." “I look at the magazines to pass the time while waiting for my appointment at the dentist’s office.” "I trying to pass the time by reading a book" "I pass the time by eating, sleeping and reading."