The idea of sundial is well known for ages, but here comes the revolution! Sundial which display digits!
That shape has been mathematicaly designed to gets you accurate time without using any kind of electrical power sources.
It works from 10 AM to 4 PM with 20minutes resolution.
All you need to do is print five parts and stick them together:
Gnomon(N/S) part 1
Gnomon(N/S) part 2
Notice that gnomon parts comes with two types: if you're living on Northern hemisphere of Earth print parts with letter N,
for Southern hemisphere parts are marked with letter S.
To connect all the pieces you will also need:
--- an (empty !) jam jar
--- 3x M6 screws, flat head, length = 20 mm
--- 1x M6 screw, flat head, length = 50 mm
--- 4x M6 nuts
--- 4x M6 washer, outside diameter < 14mm
Print it, calibrate for your position and use anywhere you want!