Works out the correspondence between currentTimeMillis and nanoTime

Time.java

import java.util.Arrays;

public class Time {

    public static void main(String[] args) throws InterruptedException {
        int n = 100;
        int[] before = new int[n];
        int[] after = new int[n];

        // warmup
        for (int i = 0; i < n; i++) {
            go(before, after, i);
        }
        for (int i = 0; i < n; i++) {
            go(before, after, i);
        }

        Arrays.sort(before);
        Arrays.sort(after);

        int beforeBound = before[n / 2];
        int afterBound = after[n / 2];

        int offsetWithinMilli = (beforeBound + afterBound) / 2;
        System.out.println(offsetWithinMilli);

        long millis = System.currentTimeMillis();
        long nanos = System.nanoTime();

        long milliInNanos = millis * 1_000_000;
        long startOfMilli = nanos - offsetWithinMilli;

        long offset = milliInNanos - startOfMilli;

        Thread.sleep(100);

        long nanoTime = System.nanoTime();
        long currentTimeMillis = System.currentTimeMillis();
        System.out.println(nanoTime + " + " + offset + " = " + (nanoTime + offset) + " -> " + (nanoTime + offset) / 1_000_000 + " == " + currentTimeMillis);
    }

    private static void go(int[] before, int[] after, int i) {
        long milliBefore = System.currentTimeMillis();
        long nanoBefore = System.nanoTime();

        long milliAfter;
        long nanoAfter;

        while (true) {
            long milliTime = System.currentTimeMillis();
            long nanoTime = System.nanoTime();

            if (milliTime == milliBefore) {
                nanoBefore = nanoTime;
            } else {
                nanoAfter = nanoTime;
                milliAfter = milliTime;
                break;
            }
        }

        if (milliAfter - milliBefore > 1) return;

        before[i] = (int) (nanoBefore % 1_000_000);
        after[i] = (int) (nanoAfter % 1_000_000);
    }

}