For your amusement I wondered if there were any open source watch projects that would let you hack BiP for example.  
Surprisingly I found a bunch of watch projects – only one looked like it had got anywhere, and it seems dormant.
AsteroidOS – the most promising because it targeted a bunch of devices, but has gone very quiet
OSWatch – no updates after 2014
Then I found a bunch of DIY everything projects
CulBox – a kickstarter that failed to launch
Instructables –  outstanding:  Smooth finish, and delicate in size.   My favorite by far… based on an Arduino.  Notice the cool way the strap is joined to the case.
Not exactly a DIY watch – Blocks – seems to have failed.