Sure - you have 5 greetings to choose from, among them the standard (plays during "on hours") and "off hours" which, as the name implies, plays off hours.
Associate the call handler or subscriber in question with a schedule that defines 8:30am to 5:30pm as active and then activate and record the off hours greeting (The standard greeting is always active). Calls that come i from 8:30am to 5:30pm will hear your standard greeting and calls outside that time will hear your off hours greeting.
also, here's a link to all the admin guides for all versions of Unity (you didn't indicate which version you're at):
http://www.cisco.com/en/US/products/sw/voicesw/ps2237/prod_maintenance_guides_list.html
you'll find more information about configuring subscriber and call handler options for greetings and trasnsfer rules etc... in there.
