Elementary my dear Watson:
- Oct 12 1999 is exactly 80 days before 1-1 2000.
- For some people the year 2000 was the end of the world
- As we know, it takes 80 days to go around the world.
- So oct 12 1999 was the last possible day to go around the world.
- As we know internet is wrapped around the world.
- So packets (and also cookies) travel around the world.
- The expiration date of Oct 12 1999 is the symbolic last day a packet could be send.
- There is no need to send it later than this date.
- So this is the symbolic date for do not expire.