How to unit test Scheduler using NUnit
class ProductionDateTimeManager : IDateTimeManager
    {
        public DateTime GetCurrentDateTime()
        {
            throw new NotImplementedException();
        }
    }

    public class Scheduler
    {
        public Scheduler(IDateTimeManager dateTimeManager)
        {
            this.DateTimeManager = dateTimeManager;
        }

        public Scheduler()
        {
            this.DateTimeManager = new ProductionDateTimeManager();
        }

        public void Execute()
        {
            DateTime current = DateTimeManager.GetCurrentDateTime();
        }

        private IDateTimeManager DateTimeManager { get; set; }
    }

public interface IDateTimeProvider
{
    DateTime CurrentDateTime { get; }
}

public class DateTimeProvider : IDateTimeProvider
{
    public DateTime CurrentDateTime
    {
        get { return DateTime.Now; }
    }
}

public class TestDateTimeProvider : IDateTimeProvider
{
    public DateTime CurrentDateTime { get; set; }
}

public class Scheduler
{
    private readonly IDateTimeProvider DateTimeProvider;

    public Scheduler(IDateTimeProvider provider)
    {
        DateTimeProvider = provider;
    }
}