Java clock accuracy on Windows: 15-16ms

Question

The clock accuracy on Windows seems to have changed lately. I remember it being very accurate since Windows 7 (1 ms resolution), and now the time hops in steps of 15 to 16 ms. I noticed due to some (albeit poorly written) unit tests failing (tests checked that some time elapsed between writing and reading records).

This affects following implementations:

System.currentTimeMillis()
LocalTime.now()
LocalDateTime.now()
ZonedDateTime.now()

I'm well aware that elapsed time is to be measured using System.nanoTime() deltas, but I'm wondering nevertheless if I missed something that changed back the clock resolution to 15-16ms.

Environment:

Windows 10 version 1709
JRE/JDK 1.8.0_171-b11 (64-bit)

Did anyone notice this as well? What could be the reason for this resolution change?

EDIT:
Test code to check this behaviour (see output: time changes and number of samples before change)

@Test
public void testCurrentTimeMillis() {
    test(() -> System.currentTimeMillis());
}

@Test
public void testNanoTime() {
    test(() -> System.nanoTime());
}

@Test
public void testLocalTime() {
    test(() -> LocalTime.now());
}

@Test
public void testLocalDateTime() {
    test(() -> LocalDateTime.now());
}

@Test
public void testZonedDateTime() {
    test(() -> ZonedDateTime.now());
}

private <T> void test(Supplier<T> timeSupplier) {

    int samples = 20;
    String lastTimeString = null;
    int count = 0;
    while (samples > 0) {

        count++;
        String timeString = timeSupplier.get().toString();
        if (!timeString.equals(lastTimeString)) {
            System.out.println(timeString + " (" + count + ")");
            lastTimeString = timeString;
            count = 0;
            samples--;
        }
    }
}

Can you turn on and check again? You might have to restart system. — Shanu Gupta, Apr 25 '18 at 13:19
Possible duplicate of [Timer accuracy in java](https://stackoverflow.com/questions/1245388/timer-accuracy-in-java) — Ole V.V., Apr 25 '18 at 13:31
@OleV.V. the other question dates back to 2009, back then such inacurracies were common on 32-bit Windows platforms. My case is a recent 64-bit Windows platform, where the problem resurfaced. — Peter Walser, Apr 25 '18 at 13:37
@ShanuGupta developer mode activated & restarted - same effect. — Peter Walser, Apr 25 '18 at 13:41
I’ve searched a bit around in vain. You can probably do better. Anyway IIRC, I read recently that this is a Windows issue (in 2018 still) and therefore not something that can be solved in Java alone. There are some tricks to persuade Windows to giving a better resolution, but they are expensive in power consumption, so you should use them with care if at all. There is also a JDK bug open that aims at better accuracy on Windows. Disclaimer: as I said, it’s all from fragile memory. — Ole V.V., Apr 25 '18 at 13:47
Just ran the test code on a collegue's machine - he's getting 1ms resolutions (same machine build, windows version, only the latest windows security updates KBID 4093110,4099989,4093112 from 2018-04-11 not installed). Maybe the resolution has been lowered as a countermeasure to SPECTRE vulnerabilities? — Peter Walser, Apr 25 '18 at 13:57

score 1 · Answer 1 · answered Apr 26 '18 at 10:49

To get the local time with the best accuracy regardless of the platform, I came up with a solution with is based on the current time (measuring when it is increased) as an anchor, and and offset based on System.nanoTime() deltas.

Advantages:

current time with precision not depending on the platform
high accuracy (nano time)
portable (as the java.time objects can actually carry nanosecond precision).

Code (can be adapted to serve LocalTime and LocalDateTime as well):

/**
 * Exact zoned date/time, compensates for the 15-16ms leaps 
 * of milliseconds time on some Java platforms.
 */
public final class ExactZonedDateTime {

    private static ZonedDateTime anchor;
    private static long anchorNanos;

    static {
        ZonedDateTime last = ZonedDateTime.now();
        while (((anchor = ZonedDateTime.now()).equals(last))) {
            anchorNanos = System.nanoTime();
        }
    }

    private ExactZonedDateTime() {
    }

    public static ZonedDateTime now() {
        return anchor.plusNanos(System.nanoTime() - anchorNanos);
    }
}

Subsequent calls:

2018-04-26T12:51:02.293079632+02:00[Europe/Zurich] 2018-04-26T12:51:02.293524865+02:00[Europe/Zurich] 2018-04-26T12:51:02.293598126+02:00[Europe/Zurich] 2018-04-26T12:51:02.293660770+02:00[Europe/Zurich] 2018-04-26T12:51:02.293725538+02:00[Europe/Zurich]

Java clock accuracy on Windows: 15-16ms

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