> ... accurate  clock over 8 hoursunder varying temperature  ...

What they all said, plus:

Budget / acceptable cost is a significant factor in decision making.
Here the application SOUNDS like it should allow reasonable cost but
your questions make it sound like low cost is important.
What is an acceptable all up cost?

There are many possible solutions, but simple "ovening" and a half
tolerable crystal to start would be a lot easier than many other
solutions.
Whether this is feasible depends on the energy availability but,
unless you need to build it utterly tiny and at minimum
You may wish to decide if you can lower the maximum temperature spec
as that allows a lower oven temperature.

eg a 1000 mAh LiIon will give you about 3.5 Watt hour or a constant
power of about 0.5 Watt over 8 hours. 2 x AA NimH 2500 mAh will give
you about 5 Wh.
Dissipate 1/2 Watt in an 0805 resistor on an eg FR4 PCB with thin
tracks. Apply finger pressure. (DTTAH) Report back as to whether
that's liable to be enough power to run a modetly insulated oven :-).

Tempertaure sensor and known clock drift is a viable option and less
brute force. More work to be sure you are right but more
"sophisticatd" and lower power.

Internal clock by itself is not viable.

A superb source of free temperature control is by strapping a device
to your wrist, inside of arm better than outside probably. May not be
feasible in your case :-). Adds substantially to stability of watch
time keeping accuracy.


 Russell
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