The U.S. Navy has posted a summary of the y2k problem. It describes the monumental task of coordinating every computer's operation with every other one. One may be compliant; the other may not be. How will they work together? They must be tested, one by one.
What if there are a whole lot of computers that interact with each other? One by one.
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Adding to the complexity of the year 2000 challenge are interfaces between systems. Of particular concern is the synchronization of data when the system at one end of the interface has corrected any problems and the system at the other end has not. Coordination between systems will be critical and filters may have to be used at those interfaces until both systems have corrected any problems.