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Use a crystal oscillator and run it through a divider chain or use a ready made clock/date chip. All you then need to do is to feed this signal via latches to your two micros. You do not say what sort of range you are after. One sort of computer timer uses "ticks" (pulses every .01 seconds?), so the computer has to read a very high number of ticks as a straight number. Can't remember the details exactly,but in about the year 1998, our Dec computers ran out of their 2 ^64 ticks since the conception of the operating system. Rather like the millenium bug, they started again with the time = 0000.... You could encode the time in BCD and the date in standard Gregorian calender form or any thing you fancy.