WHEN the five permanent members of the United Nations Security Council - Britain, China, France, the Soviet Union, and the United States - meet again to limit the spread of dangerous weapons, they should try to set a date for ending their own tests of the most dangerous of them all: nuclear weapons. The five just conferred for two days in Paris to begin considering proposals to rein in arms sales and the proliferation of chemical and nuclear weapons. But they left off their agenda an important proposal that could help inhibit near-nuclear countries from testing their experimental devices to see how they work - a world-wide ban on all nuclear-weapon testing. The 1963 test ban treaty only prohibits tests in space, the oceans, or the atmosphere; it allows underground tests to continue. The Bush administration advocates a gradual, step-by-step approach to end testing by the United States, Britain, and the Soviets. In perhaps a decade, the US would be ready for talks to reduce the size of tests below the present level of 10 times the yield of the Hiroshima bomb. Other such steps would follow later - until, at some distant and unspecified date, the three would end their testing when nuclear weapons were no longer needed. A quicker end is advocated by most of the developing countries of the world. They organized a conference this year to amend the 1963 treaty immediately to end all tests. This approach was vigorously opposed by Britain and the US and, like the first approach, did not even include China and France, two nuclear-weapon powers now conducting tests. Neither approach is likely to end testing in the foreseeable future. We propose a middle way: The five avowed nuclear-weapon powers - the same five who are permanent members of the Security Council - should agree on a certain date, say 1995, for ending all their tests. They should also begin negotiating a treaty to ban all tests by any country. The 1963 test ban was produced by negotiations between Britain, the USSR, and the US. China and France both thought it was aimed at them because they were at early stages in their own nuclear-weapon programs. Both conducted atmospheric tests in defiance of the 1963 treaty for years and have refused to join it since. Both are now continuing their testing underground. France has suggested that it would not be the last to stop testing. China's official position is that the USSR and the US must first end testing, production, and deployment of nuclear weapons, and make drastic cuts, before China would agree to limit or reduce its nuclear weapons. But it has recently shown flexibility in talks among the five in Paris and at the UN Security Council, and in those among the 39 members of the Geneva disarmament conference. Meanwhile, a host of factors have pushed the USSR and the US toward satisfying China's preconditions: cutbacks in nuclear deployments and weapons to fulfill agreements with allies, reduced defense budgets, and arms control agreements such as INF, START, and likely follow-ons. China may now be prepared to discuss a date to end all nuclear testing. Without such a deadline, none of the five testing countries has any incentive to prioritize its testing so it could end by a certain date. Weapons designers can continue business as usual with no need to distinguish between tests that are essential and those that are not. In that case they will always find more tests to do. If a date is set, however, the essential tests can be determined and conducted. In this country, improving safety of warheads has been urged as the top priority by a committee of experts led by Sidney Drell of the Stanford Linear Accelerator Center. If their recommendations are followed, at least in part, and a date to end testing were set, tests to develop new weapons or check the reliability of explosions could be put aside, if necessary, to test for safety against unintentional explosion. Otherwise, the weapons labs will test whichever devices are ready first. Ending testing is desirable for its own sake because it would slow further weapon development by all the nuclear powers. Given the end of the cold war, the US does not need more new designs. Ending testing would give new legitimacy to efforts by the five testing countries to persuade near-nuclear countries not to go further - India, Israel, Pakistan and North Korea, for instance. These countries are not likely to listen to "do as I say, not as I do." An announcement that testing would end by 1995 would prepare the way for a new world-wide treaty to ban all future tests, a treaty that we expect as many countries to accept as signed the 1963 test ban treaty, among them India, Israel, and Pakistan. The 1995 date would coincide with a crucial conference that will decide how long to extend the Nuclear Non-Proliferation Treaty. It codifies an international norm against more countries acquiring nuclear weapons and provides for inspections to this end. A conference last year to review its operation broke up in disagreement because developing countries were angry at the refusal of Britain and the US even to participate in negotiations to end testing. If a vote to extend the non-proliferation treaty had b een taken then, it would probably have gone against a long extension. But ending nuclear testing by 1995 would win votes among the non-nuclear countries for a long extension of the most important arms control treaty reached since World War II. The five testing countries will meet again to try to restrain the frightening spread of weapons of mass destruction around the world. Why not start by agreeing to set a date for ending nuclear testing?