Transact SQL :: Get Subquery To Eliminate Malformed Date?

I have the following query.

SELECT a.Line_Number as Line_Number,
Cast(a.election_effective_date as DATE) as election_effective_date,
Cast(a.Plan_Year_Effective as DATE) as Plan_year_Effective
FROM (SELECT aa.* FROM TEMP_Validation aa WHERE IsDate(aa.Plan_Year_Effective) = 1 AND IsDate(aa.election_effective_Date) = 1) a
Where Cast(a.election_effective_date as Date) Not Between Cast(a.plan_year_effective as Date)
and DATEADD(DD,-1,DATEADD(YY,1,Cast(a.plan_year_effective as Date)))

There is a malformed date in the Temp data, at line 4932.  This is why I created a subquery that checks if the plan year and election effective dates are dates using the "IsDate" function.  However, when I execute this I get the error "Conversion failed when converting date and/or time from character string".  If I eliminate line 4932 the error no longer occurs.  Somehow line 4932 is not being eliminated in the subquery, except if I run JUST the subquery it will not include line 4932.  What am I doing wrong on these queries and how can I correctly get the subquery to eliminate the malformed date?