I tried using

FROM

dbo.tbSales

WHERE Case When ISDATE(Firstdisbursementdate) = 1 AND LEN(Firstdisbursementdate)>= 8

Then CONVERT(DATETIME,Firstdisbursementdate,120)

Else Cast(0 as datetime) End

BETWEEN @FirstDay AND @LastDay and I still get the same error..

Msg 8115, Level 16, State 2, Line 4

Arithmetic overflow error converting expression to data type datetime.