ScheduledExecutorService very inaccurate?

Hi,

i'm using a ScheduledExecutorService to schedule a task. The problem is,
that the task gets execute about 10 - 20 percent too late, e.g. when I
schdule it with 20 s delay, it gets executed after 24 s.

Is this "normal"?

Here is the testcode:

package com.foo;

import static junit.framework.Assert.assertEquals;

import java.util.concurrent.Executors;
import java.util.concurrent.ScheduledExecutorService;
import java.util.concurrent.TimeUnit;

import org.junit.Test;

public class SchedulerTest {
    private ScheduledExecutorService s;
    private long executedAtMillis;

    @Test
    public void test1() {
        s = Executors.newScheduledThreadPool(1);
        final long currentTimeMillis = System.currentTimeMillis();
        long delay = 20000L;
        s.schedule(new Runnable() {
            public void run() {
                executedAtMillis = System.currentTimeMillis();
            }
        }, delay, TimeUnit.MILLISECONDS);
        try {
            Thread.sleep(delay * 2);
        } catch (InterruptedException e) {
        }
        assertEquals(delay, executedAtMillis - currentTimeMillis);
    }
}

When I use java.util.Timer everything is ok:

    @Test
    public void test2() {
        Timer t = new Timer();
        final long currentTimeMillis = System.currentTimeMillis();
        long delay = 20000L;

        t.schedule(new TimerTask() {
            @Override
            public void run() {
                executedAtMillis = System.currentTimeMillis();
            }
        }, delay);
        sleep(delay);
        assertEquals(delay, executedAtMillis - currentTimeMillis);
    }

Thank you.