With the Office 12 beta 1 in sight, Jensen Harris wrote an article about the Office build numbers. Apart of the usage of this story, it is nice to read 
“In Office 2003, the "11" that precedes the build number is simply to denote that Office 2003 was version 11 of Office. Similarly, the 12 in Office "12" means... well, you get it. Office XP was version 10, Office 2000 was version 9, Office 97 was version 8. You get the idea.
The most interesting thing to watch for is the first 4-digit number you encounter. In the examples above, 5608 and 3417. These are what we refer to as the "build number." Every few days during the development cycle, we compile all of the code in Office and turn it into a "build": essentially an installable version of all the work everyone's done up until that point. Eventually, a build becomes "final" and that is the one that ends up on CDs and in the store.
The 4-digit build number is actually an encoded date which allows you tell when a build was born. The algorithm works like this:
- Take the year in which a project started. For Office "12", that was 2003.
- Call January of that year "Month 1."
- The first two digits of the build number are the number of months since "Month 1."
- The last two digits are the day of that month.
So, if you have build 3417, you would do the following math: "Month 1" was January 2003. "Month 13" was January 2004. "Month 25" was January 2005. Therefore, "Month 34" would be October 2005.
3417 = October 17, 2005, which was the date on which Office 12 build 3417 started.
For Office 2003 and XP both, "Month 1" was January 2000. So, the final build of Office 2003, 5608, was made on August 8, 2003.”
Read the complete build number philosophy