SELECT 
    s.Start_Date, 
    MIN(e.End_Date) AS End_Date
FROM 
    (SELECT Start_Date 
     FROM Projects 
     WHERE Start_Date NOT IN (SELECT End_Date FROM Projects)) s
JOIN 
    (SELECT End_Date 
     FROM Projects 
     WHERE End_Date NOT IN (SELECT Start_Date FROM Projects)) e
ON 
    s.Start_Date < e.End_Date
GROUP BY 
    s.Start_Date
ORDER BY 
    DATEDIFF(MIN(e.End_Date), s.Start_Date), 
    s.Start_Date;