Question

I know that System.nanoTime() is now the preferred method for measuring time over System.currentTimeInMillis() . The first obvious reason is nanoTime() gives more precise timing and the other reason I read that the latter is affected by adjustments to the system’s real-time clock. What does “getting affected by systems real-time clock ” mean ?

Answer 1

In this case I’ve found following blog post excerpt useful:

If you are interested in measuring absolute time then always use
System.currentTimeMillis(). Be aware that its resolution may be quite
coarse (though this is rarely an issue for absolute times.)

If you are interested in measuring/calculating elapsed time, then
always use System.nanoTime(). On most systems it will give a
resolution on the order of microseconds. Be aware though, this call
can also take microseconds to execute on some platforms.

Clocks and Timers – General Overview by David Holmes

Since System.currentTimeMillis() is relying on the systems time of day clock, adjustments to the time are legitimate, in order to keep it on time.

What means adjustments here? Take for instance a look at the description of CLOCK_REALTIME from Linux:

System-wide clock that measures real (i.e., wall-clock) time.
Setting this clock requires appropriate privileges. This clock is
affected by discontinuous jumps in the system time (e.g., if the
system administrator manually changes the clock), and by the
incremental adjustments performed by adjtime(3) and NTP.