Clicking on the Search link at the top of the page will take you to the Advanced Search page, where you can enter specific search criteria.
If you want to search for posts from a particular person, just start typing his or her username into the “Search by Username” box. For example, to find posts by Diana Gabaldon, start typing “Diana”, and a list of suggested matches will pop up. Just select her name from the list.
It's not possible to search using a specific date range, but you can use the date options provided to narrow down your search quite a bit. For example, you can search for “posts from 3 months ago and older”, “posts from 1 week ago and newer”, etc.
You can choose to display the search results as threads, or as individual posts. If you are searching for all posts by a specific user, choose the “posts” option.