As you have experienced, only "some" of your computers loose noticable amounts of time. Some more than others.
There is a good and simple fix.
1 - check and replace the CMOS batteries on the problem computers. This is likely the root cause.2 - install Time Synch services on your network, as follows:
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Setup one of your computers as a "time server". Have it check with one or more of the time servers out on the Internet. These are run by Universities and other government-type institutions. Most are "freely" accessible. Then have all of your computers synch with the server at least once a day.
It is best to load the time synch software that runs as a "service", rather than an "application".
There are Time Synch products available through MS TechNet, and the appropriate O/S Resource Kits. Some O/S's come with Time Synch tools. You can also check out:
http://www.eecis.udel.edu/~ntp/