SystemTime.Now

public static class SystemTime
{
    public static Func<DateTime> DateProvider = () => DateTime.Now;
    public static void Reset()  { DateProvider = () => DateTime.Now; }
    public static DateTime Now { get { return DateProvider(); } }
}

Example use

[TestFixture]
class MyDateTimeTests
{
    private DateTime now = new DateTime(2016, 6, 5, 9, 0, 0);

    [TestFixtureSetUp]
    public void TestFixtureSetUp()
    {
        // Set SystemTime.Now = 9am Sunday 5/6/2016 to test business days
        SystemTime.DateProvider = () => now;
    }

    [TestFixtureTearDown]
    public void TestFixtureTearDown()
    {
        SystemTime.Reset();
    }
}