title: TimeSpan using a nullable date
body: <p>How can I subtract two dates when one of them is nullable?</p>
  
  <pre><code>public static int NumberOfWeeksOnPlan(User user)
  {
      DateTime? planStartDate = user.PlanStartDate; // user.PlanStartDate is: DateTime?
  
      TimeSpan weeksOnPlanSpan;
  
      if (planStartDate.HasValue)
          weeksOnPlanSpan = DateTime.Now.Subtract(planStartDate); // This line is the problem.
  
      return weeksOnPlanSpan == null ? 0 : weeksOnPlanSpan.Days / 7;
  }
  </code></pre>