21 or 22

Chazal refer to the three weeks as 21 days (see Rashi Yermiyahu 1:12 מדרש אגדה השקד הזה הוא משעת חניטתו עד גמר בישולו עשרים ואחד יום כמנין ימים שבין שבעה עשר בתמוז שבו הובקעה העיר לתשעה באב שבו נשרף הבית.)  Why is this period refers to as 21 days which omits tisha b'av from the count of the days which would make it 22 days?   

In the last post, Rav Tzaddok was cited as saying the 22 days of the three weeks correspond to the 22 letters of the aleph beis.  

The Gemarah Bava Kammah (54b) points out that the letter ט does not appear in the luchos since they were going to break ח"ו פסקה טובה מישראל.  Putting these two ideas together, the Shviley Pinchas says that all the letters require a tikkun except the ט which was never broken.  It is the fact that the ט is not broken that allows for tisha b'av to eventually be turned around into a day of simcha.  A discussed in a past post, the letter ט stands for טוּבָךְ סָתִים בְּגַוָּוךְ, the hidden goodness that is contained within.  The Ohav Yisrael (Pinchas) says this is alluded to in the form of the letter ט itself, where the head is turned inward to indicate the goodness is contained inside.  He says that is why it is called tisha b'av to indicate that in the future the ט, the hidden goodness will become apparent and tisha b'av will become the head of all the holidays.