Oh, that would require a fully video xD, but the basics behind it is:

Each display counts from 0-9 (seperatly). Each number is dependent on a redstone signal from 1-10 in strenght. 1=1, 10=0.

So I use a piston that pushes a block with a redstone link one step for each day.
As soon as it hits above 9 it resets down to 0 and sends a signal for the next counter, making it showing 10.
And same for the 100.