What the difference between currentTimeMillis and nanoTime in Java?

There are two similar methods in Java: System.currentTimeMillis() and System.nanoTime() that relate to time measurement. CurrentTimeMillis returns the current time in milliseconds from the Epoch (January 1, 1970, 00:00:00 GMT) and nanoTime returns a nanosecond-precision time related to some elapsed time. So you can measure time intervals using both methods but estimate offset related to common begin only with System.currentTimeMillis() . Let’s go deeper..

Lets specify levels of difference:

• Estimated value | Epoch time vs Escaped time
• Estimation unit | Millis vs Nanos
• Clock type | Realtime vs Monotonic

Under the hood HotSpot JVM (for POSIX) use clock_gettime (https://linux.die.net/man/3/clock_gettime) with two different arguments:

• CLOCK_REALTIME for System.currentTimeMillis()
• CLOCK_MONOTONIC for System.nanoTime()

jlong os::javaTimeMillis() {
  struct timespec ts;
  int status = clock_gettime(CLOCK_REALTIME, &ts);
  assert(status == 0, "clock_gettime error: %s", os::strerror(errno));
  return jlong(ts.tv_sec) * MILLIUNITS +
    jlong(ts.tv_nsec) / NANOUNITS_PER_MILLIUNIT;
}

jlong os::javaTimeNanos() {
  struct timespec tp;
  int status = clock_gettime(CLOCK_MONOTONIC, &tp);
  assert(status == 0, "clock_gettime error: %s", os::strerror(errno));
  jlong result = jlong(tp.tv_sec) * NANOSECS_PER_SEC + jlong(tp.tv_nsec);
  return result;
}

You can find this in https://github.com/openjdk/jdk/blob/4f529f8c232b4082aa4aa39766bcf42b09885ee4/src/hotspot/os/posix/os_posix.cpp#L1412

As we know the difference between CLOCK_REALTIME and CLOCK_MONOTONIC:

CLOCK_REALTIME
System-wide realtime clock. Setting this clock requires appropriate privileges.

CLOCK_MONOTONIC
Clock that cannot be set and represents monotonic time since some unspecified starting point.
[Linux Die Net]

And also:

Another difference is CLOCK_REALTIME can jump forward or backward according to NTP. By default, NTP allows the clock rate to be speeded up or slowed down by up to 0.05%, but NTP cannot cause the monotonic clock to jump forward or backward.
[StackOverflow]

What is more important is that CLOCK_MONOTONIC are guaranteed to be monotonic only on the same core.

Mark Andreev, SWE @ Conundrum AI