How can I get the max of a cast (Hour,minute,second) using Pivot ?

SELECT t1.EmployeeID, t1.OvertimeDate, t1.Duration, t1.AchievedDuration, t2.TimeOverTimeIn, t2.TimeOverTimeOut
FROM Emp_OverTimeScheduler AS t1  INNER JOIN (
    SELECT enrollnumber, [wday],[wmonth],[wYear], [4] as [TimeOverTimeIn],  [5] as [TimeOverTimeOut]
    FROM (
        SELECT whour, wDay, InOutMode, EnrollNumber, wMonth, wYear, wMinute, wsecond
        FROM Emp_AttendanceByDevice where InOutMode in (4,5)
    ) AS DT

    PIVOT( MAX( cast(whour as char(4))+':'++cast(wMinute as varchar(2))+':'++cast(wSecond as varchar(2)) )
    FOR InOutMode IN([4],[5]))
    AS DT
    ) AS t2 ON t1.EmployeeID = t2.enrollnumber AND day(t1.OvertimeDate) =  t2.wDay and MONTH(t1.OvertimeDate)= t2.wmonth
    and YEAR (t1.OvertimeDate)=wYear
    --group by overtimedate,
   -- and month(t1.OvertimeDate)= t2.wmonth
    where EnrollNumber=27 and MONTH(overtimedate)=2

PIVOT( MAX( cast(whour as char(4))+':'++cast(wMinute as varchar(2))+':'++cast(wSecond as varchar(2)) )