I am plotting out a sci-fi setting, and trying to figure out the consequences of a non-conventional form of time travel.
In the story, time travel has been achieved by means of temporal displacement. The principle is simple; the traveller enters a capsule, and everything inside of that capsule gets teleported to an earlier point of time. The traveller can then do whatever he wants in that past for a set duration, before returning to the capsule and going back to his own time.
This time travel operates on a rubber band effect; the traveller and all his belongings are temporarily moved into another timeline, and then later brought back to his original timeline. No matter what he does in the past - there is no effect on the original timeline.
As such, in the universe where we all exist, it is only possible to "leave", and never to "arrive". From a philosophical point of view, every time traveller arrives in an exact duplicate of the universe at a certain moment of time, which then diverges from that point onwards. As soon as he leaves, that universe effectively disappears. The displacement in time is strictly temporary.
Due to conservation of matter, though, absolutely everything that goes must return to it's original universe.
It's impossible to leave anything behind, and impossible to bring anything extra back. All displaced matter will be teleported back to the capsule once the duration of the temporal displacement ends.
The only thing that can change is the structure that the matter is arranged in.
Unfortunately, this teleportation must happen on a particle by particle basis. This doesn't work well with humans.
Let's say a time traveller eats and digests an apple while in the past. When he leaves the past, all matter that used to be that apple - including lumps of his cells and blood - will be left behind in the past. Therefore, the traveller will return with chunks of his intestines missing.
The same applies if the traveller goes to the toilet in the past - even though the waste isn't part of his body anymore, it will all be coming back with him. It gets messy.
Likewise for any oxygen in his lungs from the past, and for any skinflakes he sheds, or even any cells he picks up. Anything his body takes from the environment is gone.
After a set duration, everything will return to the capsule, and nothing else.
They want to get the most use out of this temporal displacement machine. So, with that in mind, what is the maximum duration that a time traveller could linger in the past, without suffering a messy death on his return?
Really, it depends on how much human bodies take in from their environment, and how unhealthy it would be if the matter making up their cells was suddenly 'sorted'.
Theoretically, the travellers could bring back whatever environmental suit, oxygen tanks, food supplies with them that are needed, but it costs power to run the machine. They only want to transport the very minimum amount of mass.
Edit: For clarification; the universe that the traveller arrives in is not a 'real' universe. That universe (and timeline) effectively exists for only so long as the traveller is there. Once the displacement ends, that universe disappears and all that's left behind is the capsule, and the original matter. That original matter is the only thing that can return to our world.
As such, it means that (from the travellers point of view) the universe he arrives in is completely disposable to him. Nothing that happens there will have any effect on his universe. All he cares about is coming back alive to his own universe.
Effectively, the travellers can use the displacement to research the past and bring back information, but nothing else.