>*...say the current time is 930am, so the cells that have a time BEFORE 9:30am yesterday should turn red. The cells that have a time AFTER 9:30am yesterday should remain unaffected.*

There is the setting to choose so right from within the conditional format menu.  

[![Conditional formatting for cells that have date/time within the last 24 hours][1]][1]


  [1]: https://i.sstatic.net/VHw6A.png