The Restaurant at the End of the Universe is one of the most extraordinary ventures in the entire history of catering. It has been built on the fragmented remains of... it will be built on the fragmented... that is to say it will have been built by this time, and indeed has been–

One of the major problems encountered in time travel is not that of accidentally becoming your own father or mother. There is no problem involved in becoming your own father or mother that a broadminded and well-adjusted family can’t cope with. There is also no problem about changing the course of history – the course of history does not change because it all fits together like a jigsaw. All the important changes have happened before the things they were supposed to change and it all sorts itself out in the end.

The major problem is quite simply one of grammar, and the main work to consult in this matter is Dr Dan Streetmentioner’s Time Traveller’s Handbook of 1001 Tense Formations. It will tell you for instance how to describe something that was about to happen to you in the past before you avoided it by time-jumping forward two days in order to avoid it. The event will be described differently according to whether you are talking about it from the standpoint of your own natural time, from a time in the further future, or a time in the further past and is further complicated by the possibility of conducting conversations whilst you are actually travelling from one time to another with the intention of becoming your own mother or father.

Most readers get as far as the Future Semi-Conditionally Modified Subinverted Plagal Past Subjunctive Intentional before giving up: and in fact in later editions of the book all the pages beyond this point have been left blank to save on printing costs.

The Hitchhiker’s Guide to the Galaxy skips lightly over this tangle of academic abstraction, pausing only to note that the term ‘Future Perfect’ has been abandoned since it was discovered not to be.

-The Restaurant at the End of the Universe