123Splat:
at a minimum, you need to shift out 33 bits: largest time value displayed= 23.59.59,, 2+3+5+9+5+9= 33.
That is where I lost you.
I'm assuming that an output pin on the shift register is used to control a light bulb.
i.e. set it 1 turn the light goes on, set it 0, turn off the light.
You would need 2+9+5+9+5+9 = 39 bits not 33 bits to control all 39 lights.
In other words just because 23:59:59 is the largest time displayed you still have to be able turn on the lights for ones digit of 4,5,6,7,8,9
In the bigger picture if I were building something like this I'd use a ws2812 led strip.
WS2812 leds and strips are way cool for controlling lots of leds as they are easy to use and the wiring is very simple.
Get the 1M 60 led strip.
Cut the strip to size, or cut into segments and arrange vertically as desired -
If multiple vertical segments are used, simply run wires between the segments.
The library will let you address all the individual LEDs and make them any color or brightness you want.
Only a single pin is needed and no other h/w.
You could have it up and running very quickly.
You could put it in frame behind the translucent plastic used in flourscent drop ceilings.
--- bill