将varchar转换为datetime时出现“超出范围”错误

Question

简而言之，这是我的查询和错误消息：

SELECT  cs.CaseId, cs.ClientId, 
    DATEDIFF(dd, SentD.answer, FirstGrpAct.StartDate) AS NoOfDays 
FROM    vCases Cs 
INNER   JOIN (<...>) AS FirstGrpAct ON FirstGrpAct.CaseId = Cs.CaseId 
INNER   JOIN #tmpProgramAnswers Ref     ON Ref.CaseId = Cs.CaseId 
INNER   JOIN #tmpProgramAnswers SentD   ON SentD.CaseId = Cs.CaseId 
WHERE   DATEDIFF(dd, FirstGrpAct.StartDate, '2012-04-02') <= 0

  

错误：消息242，级别16，状态3，行1转换varchar   数据类型为日期时间数据类型导致超出范围的值。

SELECT  
Cs.CaseId, cs.ClientId, 
sentd.Answer,
FirstGrpAct.StartDate,
(case when isdate(SentD.Answer) = 0 then '9999999' else SentD.answer end ),
DATEDIFF(dd, (case when isdate(SentD.Answer) = 0 then '1900-01-01' else SentD.answer end ), FirstGrpAct.StartDate) AS NoOfDays 
--DATEDIFF(dd, SentD.answer, FirstGrpAct.StartDate) AS NoOfDays 

结果很奇怪。没有9999999返回。 我甚至在主查询中放置了前1名，但得到了同样的错误。

请指教。 请多谢，