A .Net assembly has a version that consists of four numbers:

• Major version
• Minor version
• Build number
• Revision number

Major and Minor should be self-explanatory.  Revision says which version of the daily build we are dealing with.  Now the really interesting part is the Build number.

Microsoft apparently adopted a strategy of constructing the Build number like this:

• year (y)
• month with leading zero (MM)
• day with leading zero (dd).

This makes it easy to identify the day on which the assembly was built.  The build number for today would be 50608.

Should there are any Yes fans at Microsoft, then I'd be sure that the RTM build of Windows Blackcomb would be made on 25th January 2009: Build number 90125 ;-)