Time travel is a tricky business. Brave is the GM who decides to take on board a campaign involving cause and effect or indeed effect and cause. Often used as a plot device, time relocation is a handy way to place characters back in the good old days which often turns out to drop them into the great mythological stories of a given world. For richer narratives like Lord of the Rings, Warhammer Fantasy or Cthulu this is an opportunity to dabble in world creation or glimpse the gods - not that that would make for a a very long game in Cthulu mind you. Moreover I would say it's a more appropriate environment to play high level characters and become part of the Mythology of a system itself; a chance to become part of Lore.
However, as tempting as it is to run such a scenario, role players will insist on doing their own thing. Off roading in temporal sensitive games can require the GM to take an overly heavy hand jus to keep a world intact. Whilst there is the argument that history cant change - attempts to achieve goals that a party is fixated on, such as the assassination of Hitler lets say, may result in repetitive play and whilst fun, are basically a dead end. Paradoxes proliferate if players start to meet themselves (always embarrassing) and create duplicates of items. Things will also spiral when they start to bring other time machines back through time.
There is a more forgiving option of sending players into the future where actions cant be judged in practical terms but there is always the possibility of bringing back knowledge or items that may unbalance a game. But also, to be fair, there is little point in running a future time travel campaign that has no interaction with the present - it may as well just happen somewhere else as far as plot is concerned.
And there is also the eternal issue of temporal ticks in the real world. Whilst we are all time travelers, spare a thought for the D&D game that has been running for 30 years ! If any of us got caught in that particular time trap, it would still be a first level adventure after the first year and then 29 more of arguing...