WHow to add time or date dimension in a cube

I am working with Northwind database and created ordercube.which has orders table as a facttable and customers,order details suppliers table as dimensiontables.I could build cube on orders table successfully but while building a message pop up to create time dimension which was not created automatically.Now my Orders table has date related coulmns in it how do I get that as time/date dimension in my cube?Please tell me how to add any new dimension later on after creating cube?and see that in browser I can not see my newly added dimension in cube designer???how to do this?