🚀 go-pugleaf

If you want centuries, go mechanical.  Use an odometer as timekeeper,
at a rate of one click per day.  Odometers donot reset to a strange
number after a hiccup.  You drive it from bimetal springs, like the
ones in a wall thermostat.  You need one that tracks the average daily
temperature and adjusts to the annual variation, i.e. a time constant of
a few days.  A block of soapstone will do nicely.  You need another with a
time constant of about 4 hours, that tracks the daily temp curve.  The
odometer body is mounted on the first spring, the second carries a pawl
arm that provides hysteresis and usually gets you one click per day.
It is comparable with the 10 ppm accuracy you can get from a quartz clock
and has better long term stability.

You need a reliable power source at wakeup time, such as a two component
chemical setup that is mixed when the flag falls.  A glass vial with
sulphuric acid is stable over centuries.
The odometer rolls around and breaks the vial.
For an simple "hello, world", just make a precharged lead-sulfate battery.
The battery plates are made from lead and leadperoxide and are stable
as long as the unit is sealed, so build the whole contraption into
a glass cage.  You get several months of service life once it starts,
and you have lots of power.
If you are celebrating something big, let the sulphuric acid etch away a
rod of magnesium that prevents 20 pounds of plutonium from joining
another 100 pounds.  Please start your 6 digit odometer at zero, though.

For an expert treatise on the subject, read Arthur C. Clarke's story
about the fire alarm and enjoy how he makes the timer and gets the nuclear
device to its place and to go off when the timer expires.  Arthur needs
no chemicals or odometers or built in bombs to get things going.

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torkil