If for some reason you can't change the value before you bind it to the grid, you can handle the RowDataBound event and divide the number by 1000 before displaying it.
// Handle the grid's RowDataBound event
MyGridView.RowDataBound += new GridViewRowEventHandler(MyGridView_RowDataBound);
// Set the value to x / 1000 in the RowDataBound event
protected void MyGridView_RowDataBound( object sender, GridViewRowEventArgs e )
{
if( e.Row.RowType != DataControlRowType.DataRow )
return;
// Cast e.Row.DataItem to whatever type you're binding to
BindingObject bindingObject = (BindingObject)e.Row.DataItem;
// Set the text of the correct column. Replace 0 with the position of the correct column
e.Row.Cells[0].Text = bindingObject.ProcessingTime / 1000M;
}