The clock is simple. Arduino reads the time from RTC module and displays the digits on a single 4 digit matrix display. A start up message can also be displayed.

The HDSP-2000 series matrix displays  come in red, yellow, green & various different sizes; they are often used in aircraft display panels. The MSD2313 is an analog of the HDSP 2000 manufactured by Siemens.  The display is a beautiful example of engineering featuring ceramic, glass and gold, containing 2 shift registers / driver ICs and 140 individual LED die which make up the 4 character 5x7 bit mapped digits.  

The Arduino sends serial data (28bits) to the input of the display chip (more than 1 can be linked in series). The 5 columns are driven one at a time, using a UDN2981 source driver (opposite equivalent to ULN2803 Darlington array) which can source up to 500mA per output pin.