This is a basic decoder that takes in a 2-bit input and toggles 1 of 4 bits on the output. The decoder will be used to toggle the common of each digit sequentially, any glitches in this circuit will result in intermittent flicker of the digits therefore the output should contain flip flops. Even as is there might be some flicker at the 7Segment since both the BCD to 7SEG decoder and this circuit will update on the same clock pulse. This creates a bit of a racing condition where we don't know which will change first. (Ideally we would disable all digits then update, then enable a single digit, but this adds alot of complexity)

Depending on the capabilities of the selected 555 IC, this circuit may need an additional driver circuit. (In the end it didn't).