Hi! I've written a novel in which my main character travels through time (into the future) 18 years. While the concept of time travel is necessary to the plot, the details are not. Meaning, the book isn't sci-fi, but contains elements that wouldn't normally exist in a contemporary novel, so while my readers won't (generally) be fact-checking my science, they need to know its possible, it happens, and the amount of time to do so is reasonable. That said, I wanted a generally plausible method of time travel, so I do make mention of time-dilation as it relates to time travel. Therein lies my question. Setting aside precisely "how", rather, just assuming easy future time travel were to exist (think TARDIS) - that we have the kind of thrust and vehicle available to easily approach the speed of light, and then return to earth without issue - can someone please give me the plausible amount of time that one would have to travel on board their spaceship/time machine in order to have 18 years of time pass on earth? Thank you for your calculations and consideration!