I ran into this when I had my CQ and was wondering, in vain, the same thing about the x days. If you look at LOA 20 under the last section, 13, you'll see the implementation schedule:

13.
Duration and Implementation Schedule
This LOA will become effective upon its date of signing and remain in effect concurrent
with the PWA subject to the following systems implementation timeline. In each
instance where a provision requires programming changes, the parties agree to explore
manual or interim implementation solutions with the goal of implementing such
provisions as soon as reasonably possible.
Section 2 A. 256. distinct PBS/PCS designator for MAC rotations
Rotations beginning in June 2010 bid period or earlier
Section 8 F. 3. deadhead seating assignment changes
Rotations beginning in August 2010 bid period or earlier
Section 16 A. 16. MED designator
Rotations beginning in August 2010 bid period or earlier

Section 23 D. 10., P. 7. a., T. 3. b. and Z. 4. a.-b. PBS changes
Rotations beginning in January 2011 bid period or earlier

Section 23 G. 5. e. and 23 K. 1. g. waiver of recovery flying changes
Rotations beginning in March 2010 bid period or earlier
Section 23 L. 8. and 9. reroute pay change
Rotations beginning in March 2010 bid period or earlier