In agent configuration page, you can choose pattern as "Recurring" then select frequencies like Daily  weekdays, start time , time zone etc. Below picture might help. 

If you use Standard agent with recurring frequency you can achieve your requirement mostly. In this mode set Max records for agent to read from agent queue and process at one go. After finishing these many queue entries agent will automatically go for sleep. Then continue processing other items of queue till it finishes. The only catch here is agent will stop once it finishes all queue items in queue and not at 9AM as your requirement is. You can try running this agent on more than one node to finish it fast or set queue entries to finish before actual business starts.

Would the below workaround work:

1. Push the queue items to some DB table

2. Create an advanced agent to run periodically. Check for the time in the Adv agent if its before 9:00 and After 06:00 PM

3. Call standard agent from the advanced agent only if the above constraint is met.

4. Delete the DB record after processing is complete

Sounds good approach but agents are individual process, how can an agent can be called from another agent ? 

Never tried this approach but Pega Engine API has AgentUtils class that can be used to start, stop agents when you need. See if that helps you.

http://<host>:<port>/prhelp/javadocs/index.html

-Rajesh'