Question

I got an interesting “time-travel” problem today, using the following code:

for (int i = 0; i < 1; i++){
    long start = System.currentTimeMillis();
    // Some code here
    System.out.print(i + "\t" + (System.currentTimeMillis() - start));
    start = System.currentTimeMillis();
    // Some code here
    System.out.println("\t" + (System.currentTimeMillis() - start));
}

And I got the result

0   15  -606

And it seems that it is not repeatable. Anyone has any clues on what happened inside during the running time? Just curious…

New edit: I used a small test to confirmed the answers below. I run the program and change the system time during the run, and finally repeat the “time-travel”:

0   -3563323    163

Case closed. Thanks guys!

More words: both currentTimeMillis() and nanoTime() are system-timer based, so they will be not monotonic if the system timer is updated (turned back, specifically). It is better to use some internet-based timer for such cases.

Answer 1

System.currentTimeMillis() depends on the system time. So it could be modified by third party systems.

For measuring time is System.nanoTime() the better option.