VS 2008 Deleting For A Particular Date?

i am using vb.net 2008 and sql server 2005 i have read that always use parameteres for adding / deleted but this is very odd.if i delete a set of records using parameters it doesnt delete.i have used

mCmd = "DELETE From TaxStructure WHERE EffDate = @mDt"
cmd = New SqlCommand(mCmd, MyConConnection)
cmd.Parameters.AddWithValue("@mDt", dtPkEffDate.Value.Date)
cmd.ExecuteNonQuery()
MyConConnection.Close()

this does not delete the data as it seems the date is not found but if i use this :

mCmd = "DELETE From TaxStructure WHERE EffDate = '" & dtPkEffDate.Value.Date & "'"
cmd = New SqlCommand(mCmd, MyConConnection)
'cmd.Parameters.AddWithValue("@mDt", dtPkEffDate.Value.Date)
cmd.ExecuteNonQuery()
MyConConnection.Close()

this works......now what am i doing wrong in the parameters.its the same sometimes for selecting the records too...somewhere it works somewhere it doesnt.