8x8 led backpack (make display go blank) help required.

I have x4 8x8 led backpacks (Adafruit) (HT16K33 driver )running with a scrolling message. I wanted the display to go blank at a set point. I wrongly assumed that setBrightnes (0) would mean no light from the display but obviously I now realise this isn't so. Within the sketch I wanted the display to run for a set period but then dim bit by bit (say over 24hrs) at which point go out until a trigger sets it back to full brightness.

I've looked and looked for examples but can't find any.
Can anyone please advice me on this.

Dim from full brightness (15) down to (o) and then to (off) over 24hrs. Trigger high back up to full brightness (15) and then loop.

Any help would be greatly received.

If you have 0 to 15 as brightness levels, then you could drop down one level every 24/16 hours = 90 minutes (5,400,000 milliseconds).
0-15 indicates MAX7219, is just a simple command to the intensity register to change the level.
capture millis() at the top of loop, compare to the prior saved time of a change, when the 90 minutes is reached, change the intensity and update the prior saved time for the next comparison.

Many thanks The led back packs use the HT16K33 driver. Thanks for the Math and yeah that's the sort of thing I'm looking for. How do I get the display to go off though?

I'm new to all this and I've only done two projects thus far with my Arduino uno. This project is much more complex (for me) as I have limited knowledge and I have to to piece tutorials together and pull out elements of what I require.

First project completed. http://youtu.be/qqGpVnpPxVs

If at all possible could you point me to sketches/projects that might enable to get my head around your suggestion and my requirements. I've searched the help files and unless you know what your looking for then it's difficult get the results.

Many thanks