First, the date you mentioned would be about right for a Panda problem reference [webmasterworld.com]
Here's a thread more specific to your question: Google's AdSense Farm Update Was a Re-ranking - NOT a Penalty [webmasterworld.com] (early on, Panda was called the "Farm" update and was thought to target Made For Adsense "content farms"0
Not every ranking drop is a penalty - although many webmasters do use that word rather casually and I appreciate that it can cause confusion.
The challenge you face is understanding whether it's a Panda demotion (a new re-ranking algorithm) or an actual penalty. Yes, both can cause significant losses in traffic and rankings, but the way the two situations are activated is quite different. Google tells us Panda is their attempt to measure "quality" via algorithm.
Panda is not about guideline violations - those bring about penalties on a specific site basis. Instead, Panda is a re-ranking algorithm that evaluates all sites and tags some as being lower quality - sites that they feel ranked well for all kinds of legitimate SEO reasons, but whose content did not measure up to their quality criteria.
We don't know exactly what all those criteria are, but the threads here are full of learnings gleaned over the past 11 months. If you would like to study up on things, you can start here:
Panda Farm Update Directory - key discussions [webmasterworld.com]