I know you can move the time marker one measure by using Ctrl+PageUp or Ctrl+PageDown.
 
But is there keystroke combination that will advance it by one beat?

not that I can think of. if your using the PRV, you can try using the TAB key to go to the next note.
not sure if this helps.

Here are a couple of CAL scripts which may be bound to keystrokes.
 
Now+1Beat.CAL
;; increments now by one beat
;; Name: Now+1Beat.CAL
(= Now (makeTime (meas Now) (+ (beat Now) 1) (tick Now)))


 
Now-1Beat.CAL
;; decrements now by one beat
;; Name: Now-1Beat.CAL
(do
   (int m (meas Now))
   (int b (beat Now))
   (int t (tick Now))
   (if (< Now (makeTime 1 2 0))
       (= Now 0)
      (if (> b 1)
          (= Now (makeTime m (- b 1) t))
         (= Now (makeTime (- m 1) (beat (- (makeTime m b 0) 1)) t)))))

 
I use CAL scripts I've downloaded some time ago to do this, but I can't find their original source anymore.
 
However, if one beat is a quarter note, the first CAL script to move one beat (quarter note) forward is:
 
(do
(dword QuarterNote TIMEBASE)
(+= Now QuarterNote)
)
 
To rewind now time by one beat (quarter note):
 
(do
(dword QuarterNote TIMEBASE)
(if (>= (- Now QuarterNote) 0) (-= Now QuarterNote) (= Now 0))
)
 
Copy/paste these lines into 2 text files (i.e. "One beat forward.cal" and "One beat rewind.cal") and bind them to your favourite keys via:
 
Preferences>Customization>Keyboard Shortcuts
 
 

Ah, Steve was faster... 
 

Perfect.  Thank you.