error conversion failed when converting date and/or time from character string

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Gridview

SqlDataSource

errors

i have this gridview, and i use sqldatasource

this is my select command

SELECT Date.dateAvailable FROM LessonDate INNER JOIN Date ON LessonDate.dateID = Date.dateID WHERE (LessonDate.lessonID = @lessonID)

however when i want to update the dateAvailable column

UPDATE       Date
  SET                dateAvailable = @dateAvailable
  FROM            Date INNER JOIN
                           LessonDate ON Date.dateID = LessonDate.dateID
  WHERE        (LesssonDate.dateID = @LessonDateID)

however it when i click on excute query it give me error conversion failed when converting date and/or time from character string

so what wrong?