Time Provider Ambient Context for mockable date times

TimeProvider.cs

public abstract class TimeProvider
{
    private static TimeProvider current;
    static TimeProvider()
    {
        TimeProvider.current =
            new DefaultTimeProvider();
    }
    
    public static TimeProvider Current
    {
        get { return TimeProvider.current; }
        set
        {
            if (value == null)
            {
                throw new ArgumentNullException("value");
            }
            TimeProvider.current = value;
        }
    }
    public abstract DateTime UtcNow { get; }
    public static void ResetToDefault()
    {
        TimeProvider.current =
            new DefaultTimeProvider();
    } 
}

public class DefaultTimeProvider : TimeProvider
{
    public override DateTime UtcNow
    {
        get { return DateTime.UtcNow; }
    }
}