Probably everyone knows how to build a digital clock. Take a simple microcontroller like PIC16F628A and connect a bunch of LEDs to pins. The rest is up to software.
Next task is what enclosure to use. If you don’t want to to build one, there are many options to use something ready. Like in this project Victor used an old enclose from broken laptop power supply. Made holes for LEDs and buttons and got pretty good looking digital clock.