System.reactive timer - how do I change the interval on the fly?

public class Class1
{
    public IObservable<long> Timer;
    public IDisposable       TimerSub;
    DateTime                 NextTime = new DateTime(0);

    public Class1()
    {
        this.NextTime = new DateTime(0);
        this.Timer    = Observable.Timer(TimeSpan.FromSeconds(1));
        this.TimerSub = this.Timer.Subscribe(x => DoWork());
    }

    private void DoWork()
    {
        bool canWork = (this.NextTime.Ticks != 0);
        this.NextTime = this.CalculateFutureDate();

        // change the timer interval to  "this.NextTime - DateTime.Now"
        // how do I do this?

        if (canWork)
        {
            // Do some work stuff...
        }
    }

    private DateTime CalculateFutureDate()
    {
        return DateTime.Now.AddDays(1);
    }
}