Unit Testing: DateTime.Now

I have some unit tests that expects the 'current time' to be different than DateTime.Now and I don't want to change the computer's time, obviously. What's the best strategy to achieve this?


-------------Problems Reply------------

The best strategy is to wrap the current time in an abstraction and inject that abstraction into the consumer.

Alternatively, you can also define a time abstraction as an Ambient Context:

public abstract class TimeProvider
private static TimeProvider current =

public static TimeProvider Current
get { return TimeProvider.current; }
if (value == null)
throw new ArgumentNullException("value");
TimeProvider.current = value;

public abstract DateTime UtcNow { get; }

public static void ResetToDefault()
TimeProvider.current = DefaultTimeProvider.Instance;

This will enable you to consume it like this:

var now = TimeProvider.Current.UtcNow;

In a unit test, you can replace TimeProvider.Current with a Test Double/Mock object. Example using Moq:

var timeMock = new Mock<TimeProvider>();
timeMock.SetupGet(tp => tp.UtcNow).Returns(new DateTime(2010, 3, 11));
TimeProvider.Current = timeMock.Object;

However, when unit testing with static state, always remember to tear down your fixture by calling TimeProvider.ResetToDefault().


public void TestOfDateTime()
var firstValue = DateTime.Now;
MDateTime.NowGet = () => new DateTime(2000,1,1);
var secondValue = DateTime.Now;
Assert(firstValue > secondValue); // would be false if 'moleing' failed

Disclaimer - I work on Moles

These are all good answers, this is what I did on a different project:


Get Today's REAL date Time

var today = SystemTime.Now().Date;

Instead of using DateTime.Now, you need to use SystemTime.Now()... It's not hard change but this solution might not be ideal for all projects.

Time Traveling (Lets go 5 years in the future)


Get Our Fake "today" (will be 5 years from 'today')

var fakeToday = SystemTime.Now().Date;

Reset the date


/// <summary>
/// Used for getting DateTime.Now(), time is changeable for unit testing
/// </summary>
public static class SystemTime
/// <summary> Normally this is a pass-through to DateTime.Now, but it can be overridden with SetDateTime( .. ) for testing or debugging.
/// </summary>
public static Func<DateTime> Now = () => DateTime.Now;

/// <summary> Set time to return when SystemTime.Now() is called.
/// </summary>
public static void SetDateTime(DateTime dateTimeNow)
Now = () => dateTimeNow;

/// <summary> Resets SystemTime.Now() to return DateTime.Now.
/// </summary>
public static void ResetDateTime()
Now = () => DateTime.Now;

You have some options for doing it:

  1. Use mocking framework and use a DateTimeService (Implement a small wrapper class and inject it to production code). The wrapper implementation will access DateTime and in the tests you'll be able to mock the wrapper class.
  2. Use Typemock Isolator, it can fake DateTime.Now and won't require you to change the code under test.
  3. Use Moles, it can also fake DateTime.Now and won't require change in production code.

Some examples:

Wrapper class using Moq:

public void TestOfDateTime()
var mock = new Mock<IDateTime>();
mock.Setup(fake => fake.Now)
.Returns(new DateTime(2000, 1, 1));

var result = new UnderTest(mock.Object).CalculateSomethingBasedOnDate();

public class DateTimeWrapper : IDateTime
public DateTime Now { get { return DateTime.Now; } }

Faking DateTime directly using Isolator:

public void TestOfDateTime()
Isolate.WhenCalled(() => DateTime.Now).WillReturn(new DateTime(2000, 1, 1));

var result = new UnderTest().CalculateSomethingBasedOnDate();

Disclaimer - I work at Typemock

Mock Objects.

A mock DateTime that returns a Now that's appropriate for your test.

One special note on mocking DateTime.Now with TypeMock...

The value of DateTime.Now must be placed into a variable for this to be mocked properly. For example:

This does not work:

if ((DateTime.Now - message.TimeOpened.Value) > new TimeSpan(1, 0, 0))

However, this does:

var currentDateTime = DateTime.Now;
if ((currentDateTime - message.TimeOpened.Value) > new TimeSpan(1, 0, 0))

I ran into this same issue but found a research project from Microsoft that solves this issue.


Moles is a lightweight framework for test stubs and detours in .NET that is based on delegates. Moles may be used to detour any .NET method, including non-virtual/static methods in sealed types

// Let's detour DateTime.Now
MDateTime.NowGet = () => new DateTime(2000,1, 1);

if (DateTime.Now == new DateTime(2000, 1, 1);
throw new Exception("Wahoo we did it!");

The sample code was modified from the original.

I had done what other suggested and abstracted the DateTime into a provider. It just felt wrong and I felt like it was too much just for testing. I'm going to implement this into my personal project this evening.

Add a fake assembly for System (right click on System reference=>Add fake assembly).

And write into your test method:

using (ShimsContext.Create()) { System.Fakes.ShimDateTime.NowGet = () => new DateTime(2014, 3, 10); MethodThatUsesDateTimeNow(); }

Category:c# Views:1 Time:2010-03-11

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