Why Robert & Virginia plan theft at final seconds of the new 2000 millennium countdown?

But they choose theft in the final seconds of the new 2000 millennium countdown. Why so?

Because that was their window of opportunity to access the mainframe.

Virginia explained....

Now, because of the Millennium Bug, our friends at International Clearance will be running integrity tests....[snip].just before midnight, local time, they'll shut down the mainframe in each zone for 30 seconds.

Now we need those 30 seconds to patch into the mainframe. We then need 10 seconds of one-on-one time with the computer to download the "stuff".

Since the Millennium is the next night...that's when they have to do it.