SELECT StartDateTable.Start_Date, EndDateTable.End_Date
FROM (
    SELECT Start_Date, ROW_NUMBER() OVER (ORDER BY Start_Date) as rowN
    FROM Projects 
    WHERE Start_Date NOT IN 
        (SELECT End_Date FROM Projects)
) AS StartDateTable
JOIN (
    SELECT ROW_NUMBER() OVER (ORDER BY End_Date) as rowN, End_Date 
    FROM Projects 
    WHERE End_Date NOT IN 
        (SELECT Start_Date FROM Projects)
) AS EndDateTable
ON StartDateTable.rowN = EndDateTable.rowN
ORDER BY DATEDIFF(day, StartDateTable.Start_Date, EndDateTable.End_Date)
    , StartDateTable.Start_Date