It remember that is was far more than that. Without having invested too much time into this, Microsoft's market cap back then was around $270bn, and the stock

It remember that is was far more than that. Without having invested too much time into this, Microsoft's market cap back then was around $270bn, and the stock rose by around 7% on 2013-08-23, the day is retirement plans were announced. So the stock jumped close to $20bn. Historical prices according to Yahoo on 2013-08-23: closed at 34.75, previous close was 32.39.