We've recently added a new filter to Xporter's liquid processor. The date_math filter can now be used to add quantities of time to a date/time field.
For example, let's say you wanted to filter out all line items that were purchased at least three months ago. To do this, you must choose to filter orders by "Liquid", and then enter the following:
order.created_at < now | date_math: -3, 'months'
This will filter out all orders created more than 3 months ago.