How can I convert the result of System.nanoTime to a date in Java?

Question

I want to convert the result of System.nanoTime() to a date.

public void tempBan(Player p, Player banner, int timeInSeconds){
    Long timeInNano = (long) (t         


        

Answer 1

You can convert it into system time using the below code

public static long convertToUnixMs(final long timeMs) {
    final long refMonoMs = monoTimeMs();
    final long refUnixMx = System.currentTimeMillis();

    return refUnixMx + (timeMs - refMonoMs);
}
public static long monoTimeMs() {
        return System.nanoTime() / 1000000;
    }

Explanation:

System.nonoTime() is a monotonic time that increases only, it has no idea of what time it is right now, but it would only increase regardless. So it is a good way for measuring elapsing time. But you can not convert this into a sensible time as it has no reference to the current time.

The provided method is a way to convert your stored nano time into a sensible time. First, you have a timeMs that is in nano time that you would like to convert. Then, you created another nanotime (i.e refMonoMs) and another System.currentTimeMillis() (i.e refUnixMx). Then you minus refMonoMs from the timeMs, and add the reference back into it to get the sensible time back.

Answer 2

In the theory, you should not use the only System.nanotime(), but you can do a simple trick with this method in order to get nanoseconds of the current time.

public class TimeProvider{
    private final static long  jvm_diff;
    static {
        jvm_diff = System.currentTimeMillis()*1000_000-System.nanoTime();   
    }

    public static long getAccurateNow(){
        return System.nanoTime()+jvm_diff;

    }
}

Even though, you can create your own Clock implementation with this way for using high-level java data time classes.

public class HighLevelClock extends Clock {

    private final ZoneId zoneId;

    public HighLevelClock(ZoneId zoneId) {
        this.zoneId = zoneId;
    }
    static long nano_per_second = 1000_000_000L;

    @Override
    public ZoneId getZone() {
        return zoneId;
    }

    @Override
    public Clock withZone(ZoneId zoneId) {
        return new HighLevelClock(zoneId);
    }

    @Override
    public Instant instant() {
        long nanos = TimeProvider.getAccurateNow();
        return Instant.ofEpochSecond(nanos/nano_per_second, nanos%nano_per_second);
    }

}

Now we can use our clock implementation like the following:

Clock highLevelClock = new HighLevelClock(ZoneId.systemDefault());
System.out.println(LocalDateTime.now(highLevelClock));  //2020-04-04T19:22:06.756194290
System.out.println(ZonedDateTime.now(highLevelClock));  //2020-04-04T19:22:06.756202923+04:00[Asia/Baku]
System.out.println(LocalTime.now(highLevelClock));  //19:22:06.756220764

Answer 3

Unfortunately, System.nanoTime() is not what you want for this.

To quote the JavaDoc:

This method can only be used to measure elapsed time and is not related to any other notion of system or wall-clock time. The value returned represents nanoseconds since some fixed but arbitrary origin time (perhaps in the future, so values may be negative). The same origin is used by all invocations of this method in an instance of a Java virtual machine; other virtual machine instances are likely to use a different origin.

You probably want System.currentTimeMillis(), in which case you can use new Date(System.currentTimeMillis() + milliseconds) to get the date for that number of milliseconds in the future.

While you could then subtract System.nanoTime(), scale the value, and add System.currentTimeMillis() to have a similar result... since you're adding System.nanoTime() anyway and therefore have the original number of seconds, you could just use System.currentTimeMillis() directly.