Leaving aside the issue of how much accuracy you need, there are a few basic 
strategies - 

1.  Find a good clock source that can be used to supply the CPU clock (1s to 
10s of MHz).

2.  Feed a high quality 32768Hz clock to your micro and use that as the 
timebase.  

3.  Find some other time source that you can decode to get the time (NMEA 
from a GPS, WWVB, NTP, etc).

4. Count AC line crossings.

#1 can be tricky to implement if you don't have the gear. 
#2 is just a matter of buying a good crystal or better yet a TCXO.   The 
Maxim part will drift as much as a minute a year, uncalibrated.
#3 Can suck up a lot of (your) time, but should keep you within a second 
pretty easily
#4 Can work very well, and will get you to within 10s at all times.

- Robert


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