Schematic:

As you can see, there is not a whole lot to it.

I'm using an ATTiny1617 because of its built in PTC (Peripheral Touch Controller). This makes using the capacitive touch buttons fairly easy since you can use the Atmel QTouch library. The RTC was chosen because of its small size and built in oscillator.
To keep power consumption low in active mode, low current LEDs which get surprisingly bright at around 0.5mA (also pulsed at 100Hz) are used.

After some software tweaking, the watch draws ~3-4mA in active mode and ~8-12µA in sleep mode.

Case:

I designed the case in Fusion360 and then 3D printed it. It fits a standard 18mm nato strap.

To mount the PCB I made some slots into the mounting posts, which can hold M1.4 nuts. Then the PCB can be screwed in by 4 black M1.4 screws.