Thanks for the replies!  I'll have to check the datasheet on the resonator (I'm
ashamed to admit I don't know its type.  Ok it's a hobby project so I'm not that
ashamed).

Thanks for the tips.  I have to look into the resonator specs (maybe switch to a
crystal) and the routines that Dale mentioned (probably both).  The clock will
synchronize regularly to a remote computer (that's using NTP) or adjust the
amount tracked in the interrupt, so the drift is workable.  I just thought that
18 seconds over an hour to be a bit much (assuming the accuracy remains
constant).

Without knowing what type I've got at home (I'm at work now), I checked a
similarly priced resonator, right there in big ol' print, "Initial frequency
tolerance: +-0.5%", but obviously I'll have to check my part.

So for grins, I looked at a 4MHz crystal, HC49US4.  Its datasheet says its
"frequency tolerance: +-50ppm".  Is that 0.005% or about 4 seconds/day?

As always, this list is a great resource, thanks again.

Kevin

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