Click through to a very useful site. It shows you what you must do to make your spreadsheet compliant.
But . . .
How many of the tens of millions of users of PC spreadsheets will hear about this in time, and then take action?
How many will be able to fix retroactively their old data by 2000?
How many PC-based spreadsheets feed into mainframes?
If these mainframes are made compliant (hypothetical), how many of them will be made noncompliant by a noncompliant PC?
Y2K is a systemic problem. It cannot be fixed.
But click through to a web site that is trying to help.