This detailed article points out that officers could be held personally liable for damages resulting from the failure of their companies to meet the Year 2000 deadline. This deadline is December 31, 1998 if the company misses 2000. Due diigence means late 1998, leaving a year for testing.
This shows why every company promises to be ready for testing in December, 1998. It has nothing to do with an actual project's plan; it has everything to do with liability. It is a date set by lawyers, not y2k program managers.