Oops! The "30 Days Before" Wash Sales Rule
If you purchased 200 shares on 3/1/11 and sold 100 shares at a loss on 3/20/11 you would be within the 30 day period and therefore the loss would be disallowed.
In fact, this specific example would not be subject to the wash sale rules. Rev. Rul. 56-602, clarifies that shares acquired at the same time as shares sold (in other words, as part of the same lot) are disregarded when applying the wash sale rules.
An example of the “30 days before” rule is as follows:
You bought 100 shares of M stock on September 24, 2009, for $5,000. On December 18, 2009, you bought 50 shares of substantially identical stock for $2,750. On December 28, 2009, you bought 25 shares of substantially identical stock for $1,125. On January 7, 2010, you sold for $4,000 the 100 shares you bought in September. You have a $1,000 loss on the sale. However, because you bought 75 shares of substantially identical stock within 30 days before the sale, you cannot deduct the loss ($750) on 75 shares. You can deduct the loss ($250) on the other 25 shares.
I apologize for the confusion and thank Mical Bovee, CPA/PFS for catching my error.