Under the Same Moon
Time travel works in very specific ways in this setting.
Firstly, the timeline is static, which means it is impossible to change the future or the past. However, any event not explicitly covered by roleplaying events is considered undefined, and can therefore be changed freely with the assumption that that constitutes a stable time travel event.
Secondly, all travel to the future or the past takes place as part of a stable time loop (with one exception, below). A stable time loop is an event where time travel doesn’t change past or future events because those events already included the presence of one’s travel through time. These are often facilitated by having one’s future self as an Ally (See Allies, page 36 of GURPS Basic Set – Characters – but ask permission before taking it).
Thirdly, any action that would change events that have already been defined by game sessions, directly or indirectly (but ignoring the butterfly effect), simply fails. Because this action would result in a paradox, the universe simply rewinds to the moment right before the action began, and the GM informs the player that that action is impossible because it would create a paradox.
Finally, there is one way it is possible to change the past: short jumps backwards. If the trip back in time does not exceed ten minutes, then the traveller is able to go back and change the past in some small way: transferring information, or a warning, to one’s past self, for instance. However, this future self has mere seconds to act before the paradox-correcting reflexes of the universe scratch that itch and remove them from existence.
Time travel will be facilitated in this campaign by as-yet unrevealed means. No one character will have the innate ability to travel through time on their own, however, nor will anyone be able to jump at all at the beginning of the game.