If you have an app that is designed to be used over a long period of time (say a year). To write tests you need to simulate the passage of time. I've been using this little helper to allow to simulated tests in the fourth dimension

TheDateTime.cs

// Instead of using calls like DateTime.Now, use TheDateTime.Now instead. In your production code it will default to using
// the read details and for testing you would make calls like:
//   TheDateTime.SwitchToMockDate(new DateTime(2018, 12, 3);
// and then test what the outcome would be. It assumes that you have this throughout
public static class TheDateTime
{
    static Lazy<LiveDateTimeFunctions> _liveDates = new Lazy<LiveDateTimeFunctions>(() => new LiveDateTimeFunctions());
    static Lazy<MockDateTimeFunctions> _mockDates = new Lazy<MockDateTimeFunctions>(() => new MockDateTimeFunctions());

    static IDateTimeFunctions _instance = _liveDates.Value;

    public static void SwitchToMockDate(DateTime currentTime)
    {
        _instance = _mockDates.Value;
        _instance.SetNow(currentTime);
    }

    public static void SetNow(DateTime value)
    {
        SwitchToMockDate(value);
    }

    public static void SwitchToRealTime()
    {
        _instance = _liveDates.Value;
    }

    public static void AddDays(int days)
    {
        _instance.AddDays(days);
    }

    public static DateTime Now
    {
        get
        {
            return _instance.GetNow();
        }
    }

    public static DateTime MinValue
    {
        get
        {
            return _instance.GetMinimumValue();
        }
    }
}

public interface IDateTimeFunctions
{
	DateTime GetNow();
	void SetNow(DateTime time);
	void AddDays(int days);
	DateTime GetMinimumValue();
}

public class LiveDateTimeFunctions : IDateTimeFunctions
{
	public DateTime GetNow()
	{
		return DateTime.Now;
	}

	public void SetNow(DateTime time)
	{
        // Don't need this for the live version
	}

	public void AddDays(int days)
	{
        // Don't need this for the live version
	}

	public DateTime GetMinimumValue()
	{
		return DateTime.MinValue;
	}
}

/// <summary>
/// This class is used to allow for mocking mechanisms to simulate what would happen with someones training
/// and the right situations occur
/// </summary>
public class MockDateTimeFunctions : IDateTimeFunctions
{
	DateTime CurrentTime = DateTime.Now;

	public DateTime GetNow()
	{
		return CurrentTime;
	}

	public void SetNow(DateTime time)
	{
		CurrentTime = time;
	}

	public void AddDays(int days)
	{
		CurrentTime = CurrentTime.AddDays(days);
	}

	public DateTime GetMinimumValue()
	{
		return DateTime.MinValue;
	}
}