This binary number is too large to handle on machines with 4 or 8 bit storage, so it is converted to BCD.

When converting to BCD each denary digit is converted to binary separately.

The first 5 will become 0101, the adjacent 3 will become 0011 and the next 5 will also appear as 0101 while the 6 will become 0110.

The complete BCD number will be 
                      0110   0101   0101   0011   0101

Each 4 bit number can now be handled independently of the others.

Have a look at the block diagram of the digital clock.

The 7447 chip converts BCD to a form suitable for driving 7 segment displays.