Not sure if you can do this in one call handler. You may need two. Since a call handler is either open/closed. Holiday and Alternate take precedence over those two.
So what I think you need is:
DID from CUCM to Unity Call Handler 1 to start the process.
Call Handler 1: Create a schedule for call handler 1. 8:00AM to 3:30PM forwards to 2671 via the Standard Greeting. Then anything outside of this schedule will follow the closed route where you can send the call to Call Handler 2.
Call Handler 2: Create another schedule for call handler 2. 3:30PM to 5:30PM forwards to 1661 via standard greeting. Then the closed greeting for THIS handler will be what you want as the 3rd step: Forwards to After Hours CLOSED message, then Voicemail. You don't need to create any hours as calls for call handler 2 will go to closed if not in that 3:30PM to 5:30PM time frame.