From Brick Technology:

Building a mechanical Lego clock that keeps time for 10000000 years. The clock has dials to display seconds, minutes, hours, days, weeks, months, years, decades, centuries, millenia, mega-annums and galactical years (time required for the Sun to orbit once around the centre of the Milky Way Galaxy).

The first component resembles a grandfather clock with a weight-driven pendulum anchor escapement. The escapement wheel rotates 1 tooth per second. Different gear trains transmit motion from the escapement to all complications from days to years to decades.

As soon as the weight touches the ground, a rewinding motor is triggered to raise the weight and “recharge the clock”. This happens every 2 minutes. A solar powered battery fuels the energy storage for the electric rewind motor. Under a cloudless sky the solar panel generates more energy than consumed by the Lego pendulum clock. A bigger energy storage could be added to run the clock at night time. To increase solar panel efficiency the solar panel is mounted on a tilting mechanism that is connected to the 24h complication, following the sun during daytime.

Similar to an astronomical clock, this Lego timepiece features complications beyond minutes and hours. It displays units of times based on orders of magnitude of the second. Days, mean months and years are counted. The biggest unit is the “billion year display” that is basically a mechanical counter displaying years in decimals.

[Brick Technology]