Epson 32.768 khz xtal's are available from www.DigiKey.com for less than
50 cents.  Or you can take apart an old digital wrist watch and take the
crystal out.  Some watches have the crystal leads clamped like a socket
and the leads are nice and long.

32768 is a nice round number for dividing down to a second with nested
looping. From there, minutes, days, millenniums are a snap.  Accuracy
should be pretty much the same as the original wrist watch (with the
right caps on the xtal).  Some $5000 watches aren't any more accurate
than a dollar digital watch.

Parallax once had an article called Timing.doc on their bbs and website
that discusses this and the RTCC and interrupts.  Microchip AP notes
describe RTC (real time clock) and alarm clock devices.

Don