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.