Leave both windows blank to just search for keywords/username with no date limit. If you click the drop down button on the "newer than" window where it says days, weeks, months and years you will see a blank space above days. Click on that, and clear the "1" out of the other window and the system will search as far back as needed to get 200 results.

As an alternative:

You click on a user's name in a thread, which brings up their profile. Click on the "show all users posts", and it will bring up a list of posts by that person.
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"In case you ever wondered what it's like to be knocked out, it's like waking up from a nightmare only to discover it wasn't a dream." -Forrest Griffin