>>    Observation reveals that, if you trim a digital wrist watch
>>    crystal oscillator carefully you can get long term accuracies of
>>    well under 1 second per day drift. This is a low cost 32 KHz
>>    crystal which hasn't been chosen for its precision!
>>
>It's also used in an environment that maintains a relatively
>constant temperature...

I've been reading up on timekeeping recently, and ran across a bunch of
interesting information.  Here's a couple of links worth checking out.

Best of all is the Hewlett Packard application
note on the science of timekeeping.  It includes a
description of an experiment performed on some really
cheap digital stopwatches over a period of 3-4 months.
They decribe how you can model the errors in the
frequency and aging to correct the indicated time.

It's available on-line at:
http://www.tmo.hp.com/tmo/Notes/English/5965-7984E.html


The HP app note references a patent on a 'smart' watch
that updates an equation containing drift and aging
correction terms.  It looks like it would make a fun
little project for a PIC, lcd, and oven oscillator.

It's also available on-line (as most recent patents
are) at:
http://www.patents.ibm.com/details?patent_number=5%2C274%2C545


There are several other HP app notes on subjects such
as electronic counters, GPS time standards, crystal
oscillator design, etc.  Many of these would be relevant
to any discussion of high-accuracy oscillators and
timepieces.


later,
newell