We have just added a highly requested feature for exporting only a portion of your plan. It's a very common practice that on large plans, you may want to communicate just the current month or the next three months, and not all the plan that has been already completed.
On the Export menu, now you'll find a "Date Range" section which defaults to "Full". You can choose a start and end date so the exported Image or PDF will contain only the selected period.
We have also included a "Completion date" on the Spreadsheet export containing the date is was marked as completed (a non editable field provided by Asana). This is a very useful field for calculating project progress!