Corrected entry: Lister says that killing your past self will cause you to die, thus making it impossible to go back in time to kill your past self in the first place. So how was JFK able to kill himself at The End of the episode? If he killed himself, he would have still survived as he wouldn't have been unable to go back in time and kill himself in the first place, the same was as the crew survived their attack.
Corrected entry: In the episode 'Tikka To Ride', Lister tells us he has recently passed his 28th birthday. He was 25 in 'Future Echoes', meaning that all the episodes up to this point have taken place in around 3 years. However, dialogue in 'Back To Reality' and 'Holoship' (which took place before 'Tikka To Ride') says that the crew have been travelling for around 4 years.