SQLの日時フィールドから時間のみを更新します

UPDATE MyTable 
SET MyDate = DATEADD(HOUR, 4, CAST(CAST(MyDate AS DATE) AS DATETIME)) 
  

 またはこれ
 
UPDATE MyTable 
SET MyDate = DATEADD(HOUR, 4, CAST(FLOOR(CAST(MyDate AS FLOAT)) AS DATETIME))