Is it possible to block user login at certain times or dates?

Not natively. What's the use case? The only reason I can think of wanting to do this (period closing) is what the lock periods permission/functionality is for. Otherwise you're probably better off just restricting permissions/access/roles across the board rather than at specific times/days.

A possible solution is to create a scheduled script to automatically inactivate and reactivate the users every day. Inactive users won’t be able to log into NetSuite.