THE APPROACH of Dec. 31, 1999, has not brought full clarity on how much we should be worrying about the "Y2K problem"--the possibility that computers will read the new date as Jan. 1, 1900, and cease to function. Rep. Stephen Horn, who has issued periodic "report cards" on government efforts to avert the problem, this week offered a typical mix of reassurance and ambiguity. Most systems, it seems, are go, but people should be prudent and avoid relying on them; on the other hand, the air traffic control system is still described unnervingly as "playing catch-up," but Mr. Horn and FAA administrator Jane Garvey both declare they will be in the air on New Year's Eve, just to show everything is fine.

The readiness people have a delicate line to walk. Until now they have spent their time trying to jolt sleepy bureaucracies into action with tough talk about what might go wrong if banks, power companies, hospitals, government agencies and others failed to overhaul their computer systems. Now, with no time left for full-scale reprogramming, the greater danger is of setting off a system-crashing panic. "The phone system will be fine," one government official predicts, "as long as everybody doesn't pick up the phone right after midnight to see if it's still working."

Whether from prudence or procrastination, surprising numbers of Americans so far are behaving just as the worriers would like them to. In contrast to the flying administrators, some 60 percent of Americans earlier this month said they had no plans to go out of town for New Year's Eve celebrations, and 45 percent were planning to stay in their homes. There are still some large question marks, mostly concerning international networks whose overseas components may not have been fully tested, and a panic-fueled run on supplies. But the best reason not to hoard food or withdraw all your money from the bank is that you probably aren't going to need it. It remains unlikely that great rips will appear in the fabric of civilization on Jan. 1, 2000.