How to calculate Average of DateTime ?

CREATE TABLE Transactions
(
      TransID INT,
      TransDate DATETIME
);
INSERT INTO Transactions VALUES (1, '2010-08-11');
INSERT INTO Transactions VALUES (2, '2010-08-12');
INSERT INTO Transactions VALUES (3, '2010-08-14');
INSERT INTO Transactions VALUES (4, '2010-08-15');
INSERT INTO Transactions VALUES (5, '2010-08-16');
INSERT INTO Transactions VALUES (6, '2010-08-17');
INSERT INTO Transactions VALUES (7, '2010-08-18');

SELECT CAST(AVG(CAST(TransDate AS FLOAT)) AS DATETIME)
FROM Transactions