Validation in Date Time

I got a error in this line..

objsal.SalDate = Convert.ToDateTime(ds.Tables[0].Columns.Contains("SalDate"));

where objsal is object for salary table in data acccess layer...

for this line I am giving an input as

objdb.DBRetrieveSal("mk23", Convert .ToDateTime ("5/7/2008 5:23AM"));

where objdb is object in data access layer, DBRetrieveSal is function to retrieve Salary details..

I am getting error as "Invalid cast from 'Boolean' to 'DateTime'."..

Any error i ...