Can I create a formula to provide a value depending on when a timestamp falls in the day?
Assuming you have a normal Numbers Date/Time 2/11/2016 12:00:00 AM, then a simple IF() statement can be used combined with the HOUR function.

=IF( HOUR ( A2 ) &lt; 12, "Good Morning", "Good Afternoon")

To add more than just 2 options, nest your IF statement.

=IF(HOUR(A3) &lt; 5,"Early",IF(HOUR(A3) &lt; 12,"Morning",IF(HOUR(A3) &lt; 17,"afternoon",IF(HOUR(A3) &lt; 20,"evening","night"))))

Numbers will return the first TRUE value.