Can I change the current date and time for a specific process? - unit-testing

Question

Is there a way to change the date and time seen by a specific process as opposed to the entire computer (i.e. the time returned by time() in C, by DateTime.Now in C#, by datetime.now() in Python, by new Date() in JavaScript, by System.currentTimeMillis() in Java, etc.)?
For instance, is there a way to fool a program into thinking it's midnight while it's 7 AM on the rest of the computer, or to make it think it's Christmas or Leap Day while the system clock shows July 18th?
An applicable situation would be when you're writing unit tests and you want to quickly test any special behavior the program has on Christmas or on Leap Day or on the demo's expiration date, and you want to test this behavior without having to use the date changing function in the OS and interfere with programs that rely on the date being correct (much less forcing me to have superuser privileges over the computer).

Answer 1

Previously mentioned Typemock also has an API for C++, which also allows you to fake time, in C++:
TEST_METHOD(FakeCurrentTime)
{
SYSTEMTIME fakeTime;
fakeTime.wYear = 2000;
fakeTime.wMonth = 1;
fakeTime.wDay = 1;
FAKE_GLOBAL(GetSystemTime);
WHEN_CALLED(GetSystemTime(RET(&fakeTime))).Ignore();
SYSTEMTIME now;
GetSystemTime(&now);
Assert::IsTrue(now.wMilliseconds - fakeTime.wMilliseconds == 0);
}

Answer 2

I don't think you can do what you want to do in C#. There aren't any hooks that I know of to make DateTime.Now return arbitrary values out of the box. The standard way to implement unit testing that is time sensitive is to create a time provider interface. This is the approach taken by third party libraries such as Noda Time that values testing time related functions. One such implementation is below (not Noda Time compatible):
public ITimeProvider
{
DateTime CurrentTime { get; }
DateTime CurrentUtcTime { get; }
}
public DefaultTimeProvider : ITimeProvider
{
public DateTime CurrentTime { get { return DateTime.Now; } }
public DateTime CurrentUtcTime { get { return DateTime.UtcNow; } }
}
Then when you want to unit test the code you replace the DefaultTimeProvider implementation with a mock implementation. You can use your favorite mocking framework or just write something like this:
public MockTimeProvider : ITimeProvider
{
private readonly DateTime _currentTime;
private readonly DateTime _currentUtcTime;
public MockTimeProvider(DateTime currentTime, DateTime currentUtcTime)
{
_currentTime = currentTime;
_currentUtcTime = currentUtcTime;
}
public DateTime CurrentTime { get { return _currentTime; } }
public DateTime CurrentUtcTime { get { return _currentUtcTime; } }
}

Answer 3

Disclaimer, I work at Typemock.
You can use Typemock Isolator for this:
[TestMethod]
public void isLicenseExpiredTest()
{
Isolate.WhenCalled(() => DateTime.Now).WillReturn(new DateTime(2017, 5, 4));
bool result = Licenses.IsLicenseExpired();
Assert.IsFalse(result);
}
public static class Licenses
{
public static bool IsLicenseExpired()
{
if (DateTime.Now > new DateTime(2016, 5, 4))
{
return false;
}
else
{
return true;
}
}
}

Answer 4

java.time
Java 8 and later has the java.time framework built in. Supplants the old date-time classes.
Clock
Includes the Clock class. You can override this class to make your own faked current time.
A few alternate implementations are included via static methods. You can ask for a Clock that stays fixed on a single moment you specify. You can shift the current time by some specified amount. And you have the Clock report the current moment rounded to the whole second or minute and so on.