Fastest way to check a List<T> for a date

Question

I have a list of dates that a machine has worked on, but it doesn't include a date that machine was down. I need to create a list of days worked and not worked. I am not sure of the best way to do this. I have started by incrementing through all the days of a range and checking to see if the date is in the list by iterating through the entire list each time. I am looking for a more efficient means of finding the dates.

class machineday
{
 datetime WorkingDay;
}

class machinedaycollection : List<machineday>
{
public List<TimeCatEvent> GetAllByCat(string cat)
{
  _CategoryCode = cat;


  List<machineday> li = this.FindAll(delegate(machinedaydummy) { return true; });
  li.Sort(sortDate);
  return li;
}

int sortDate(machinedayevent1, machinedayevent2)
{
  int returnValue = -1;
  if (event2.date < event1.date)
  {
    returnValue = 0;
  }
  else if (event2.date == event1.date)
  {
    //descending
    returnValue = event1.date.CompareTo(event2.date);
  }
  return returnValue;
}
}

Answer 1

Sort the dates and iterate the resulting list in parallel to incrementing a counter. Whenever the counter does not match the current list element, you've found a date missing in the list.

List<DateTime> days = ...;
days.Sort();
DateTime dt = days[0].Date;
for (int i = 0; i < days.Length; dt = dt.AddDays(1))
{
    if (dt == days[i].Date)
    {
        Console.WriteLine("Worked: {0}", dt);
        i++;
    }
    else
    {
        Console.WriteLine("Not Worked: {0}", dt);
    }
}

(This assumes there are no duplicate days in the list.)