LocalDateTime.now() has different levels of precision on Windows and Mac machine

The precision is different because LocalDateTime.now() uses a system default Clock.

Obtains the current date-time from the system clock in the default time-zone.

This will query the system clock in the default time-zone to obtain the current date-time.

…

The link in this Javadoc takes you to Clock.systemDefaultZone() which states (emphasis mine):

Obtains a clock that returns the current instant using the best available system clock, converting to date and time using the default time-zone.

This clock is based on the best available system clock. This may use System.currentTimeMillis(), or a higher resolution clock if one is available.

…

Which clock Java uses can depend on a lot of things and it looks like your Mac computer has a clock with microsecond precision whereas your Windows computer has a clock with millisecond precision. I’m not aware of any way to increase the precision of a clock but you can definitely decrease the precision so that it matches across platforms.

One option is to use LocalDateTime.truncatedTo(TemporalUnit):

import java.time.Instant;
import java.time.temporal.ChronoUnit;

public class Main {

  public static void main(String[] args) {
    Instant original = Instant.now();
    System.out.printf("Original instant  : %s%n", original);

    Instant truncated = original.truncatedTo(ChronoUnit.MILLIS);
    System.out.printf("Truncated instant : %s%n", truncated);
  }
}

^{-Also see answer by Ole V.V.}

Another option is to plug in your own Clock and use LocalDateTime.now(Clock). If possible, I would use Clock.tickMillis(ZoneId) since this method returns a Clock that truncates to milliseconds.

Obtains a clock that returns the current instant ticking in whole milliseconds using the best available system clock.

This clock will always have the nano-of-second field truncated to milliseconds. This ensures that the visible time ticks in whole milliseconds. The underlying clock is the best available system clock, equivalent to using system(ZoneId).

…

Since:
9

For example:

import java.time.Clock;
// import java.time.Duration;
import java.time.Instant;
import java.time.ZoneId;

public class Main {

  public static void main(String[] args) {
    // Java 8
    // Clock clock = Clock.tick(Clock.systemDefaultZone(), Duration.ofMillis(1));

    // Java 9+
    Clock clock = Clock.tickMillis(ZoneId.systemDefault());

    Instant instant = Instant.now(clock);
    System.out.printf("Instant : %s%n", instant);
  }
}