Replacing DateTime.Now method to assist with time sensitive unit tests

DateTimeExtensions.cs

// Following on from: http://stackoverflow.com/q/7661953/235644
// 
// Observations:
// Should consider using UTC for better globalization support
//
// Usage:
DateTimeExtensions.SetNowFunction(() => date1);

/// <summary>
/// Allow custom generation of Now times.
/// </summary>
private static Func<DateTime> _nowFunction = () => DateTime.Now;

/// <summary>
/// Get the current time using the Now generator function
/// </summary>
public static DateTime Now
{
    get { return _nowFunction(); }
}

/// <summary>
/// Set the Now generator function
/// </summary>
public static void SetNowFunction(Func<DateTime> function)
{
    _nowFunction = function;
}