Controlling Time with Java Clock

Use java.time.Clock to control time in your unit tests. This article will mainly focus on usage of fixed clock and offset clock.


As a Java developer, it’s very common to write code related to date and time. Whenever you need to manage the lifecycle of some objects or keep track of the key events, you likely need some objects to represent date and time. Since Java 8, Joda Time became Java Time (JSR-310) and it is now part of the Java built-in APIs. It provides significant values to the Java community without the need to add additional dependencies. Today, I’m going to explain java.time.Clock, an essential class for controlling date/time objects in java.time, especially in tests.

After reading this article, you will understand:

  • What is a fixed clock?
  • What is an offset clock?
  • Which APIs accept clock as an input?
  • How to go further on this topic?

This article is written in Java 11, but most of the concepts should be available since Java 8.

Fixed Clock

A fixed clock is a clock that always returns the same instant. It freezes the world at a fixed moment. This is useful for testing: asserting any calculation which uses an instant T as reference. It ensures that the tests do not depend on the current clock. For example:

  • Asserting the code behavior before or after the instant T, such as testing a feature flag that will change the code behavior and will be enabled at the instant T.
  • Asserting a method which computes the result based on current instant now.
// Given a clock fixed at 2020-05-24 14:00:00
var datetime = LocalDateTime.of(2020, 5, 24, 14, 0);
var instant = ZonedDateTime.of(datetime, ZoneId.systemDefault()).toInstant();
var clock = Clock.fixed(instant, ZoneId.systemDefault());

// When asking the "now" via this clock
var now =;

// Then "now" is not now, but 2020-05-24 14:00:00

As you can see, instead of returning the actual “now”, local-date-time’s now(Clock) returns the local date time instant controlled by the fixed clock, i.e. 24 May 2020 at 14h00.

Offset Clock

Offset clock adjusts the underlying clock with the specified duration added or subtracted. This is useful for testing: asserting any behavior that requires a duration to take effect. In other words, simulating something in the future or the past. For example: asserting the cache invalidation after its time-to-live (TTL).

Here is a small class to demonstrate how the clock works: it contains multiple entries, stored inside an underlying hash-map. You can use put(String, Instant) to put more entries to the cache or using clearExpired(Clock) to clear the expired entries.

class Cache {
  static final Duration TTL = Duration.ofHours(1);
  final Map<String, Instant> cache = new HashMap<>();

   * Remove expired entries.
   * @param clock clock to control the time
   * @return expired entries being removed
  List<Entry<String, Instant>> clearExpired(Clock clock) {
    var now =;
    var it = cache.entrySet().iterator();
    var expired = new ArrayList<Map.Entry<String, Instant>>();

    while (it.hasNext()) {
      var entry =;
      if (entry.getValue().plus(TTL).isBefore(now)) {
    return expired;

  void put(String key, Instant value) {
    cache.put(key, value);

For testing, the challenge is to test the time-to-live (TTL) by making some entries cache expired. Without a clock, this is difficult to achieve. We can use Thread.sleep(long) to make the current thread sleep and wait for one hour… But this is far from ideal, because not only it increases the execution time of the tests, but it may also lead to non-deterministic scenarios in some cases. In other words, it may introduce random failures. On the contrary, using offset clock just works: it adds exactly 1 hour offset to the system default clock as shown below.

var clock = Clock.offset(Clock.systemDefaultZone(), Cache.TTL);

In this approach, we simulate the future in 1h and test the expiry successfully without any wait time. Here is how the complete assertion looks like:

// Given a cache with some entries
var instant =;
var cache = new Cache();
cache.put("k1", instant);
cache.put("k2", instant);
cache.put("k3", instant.plusSeconds(7_200));

// When removing expired entries from the cache
var clock = Clock.offset(Clock.systemDefaultZone(), Cache.TTL);
var removed = cache.clearExpired(clock);

// Then removed entries contains exactly k1 and k2
assertThat(removed).containsExactly(entry("k1", instant), entry("k2", instant));


But you may ask: which APIs accept Clock as an input parameter? Well, all the methods which create a java.time object, they all accept Clock as an alternate clock for testing. For example: Instant, LocalDate, LocalTime, LocalDateTime, ZonedDateTime, OffsetTime, OffsetDateTime, Year, YearMonth, …

Going Further

Still want to go further from here? Here are some resources that you may be interested:

You can also check the source code of this article on GitHub.


In this article, we saw how to use a fixed clock and offset clock from java.time.Clock to control the date and time in your tests. A fixed clock ensures that tests are not dependent on the current clock; an offset clock simulates an instant in the future or the past. They are useful for writing tests and avoid increasing execution time or random test failures. All Java Time classes representing a date/time accept clock as its input parameter. Interested to know more? You can subscribe to the feed of my blog, follow me on Twitter or GitHub. Hope you enjoy this article, see you the next time!