It took me way to long to learn this. Your code (and their unit tests) should inject the system clock as a dependency.

An example, let’s say you have a service that writes a record to the database with the system clock.

public void save(String userName) {
	long currentTimeMs = System.currentTimeMillis();
	User user = User.builder()
	    .name(userName)
		.updateTimeMs(currentTimeMs);
	database.save(user);
}

How would you test this? You can inject a mock database instance and use it to verify that it got a User object. Great! You can verify the username is as expected. How do you verify that tricky business rule that updateTimeMS is the “current time”?