The core set rule book on page 2 specifically stated the time frame for the base campaign was 3. I have seen nothing that declares a time period for either Twin Shadows or Return to Hoth. I've checked the online version of the rule books, and nothing...
Most of us were expecting RTH to be time period 4, with the next big box to be time period 5. Now, Han's mission is usable from 2-4, and Vader's is 1-6 (IIRC), which certainly suggests that ROTJ would end time period 6, but what should we make of all this?
Did I miss the time period declaration for RTH?