With regards to dating works... sometimes that is the purpose of including familiar brands. They help to give a piece a fixed point in time. If it is a contemporary work it allows later readers to get a better understanding of the period it was written in (in the case of Douglas Adams even people who were not alive in the 80s know that every wore digital watches back then ). If it is a period piece, it adds to the versimilitude. A good example of this in writing is Life On Mars where there are several brandnames dropped in which no longer exist but were common in the 70's. Quantum Leap (to use an example which more of you may be familiar with) also did this with the Burma Shave adverts. They place the period and location of the story as definitely 1950s and America as that brand is rarely seen elsewhere in time or space.