Time travel is generally a two form of time travel, and is usually planned. Besides being able to explore a lost time, good stories also have clear rules on how time travel happens. Common rules are wether you can change history, and if so what happens? What kind of plot device enables time travel?
The Time Scout Series (co-author with Linda Evans)
I have yet to read book #2, Wagers of Sin. A time travel series. Time travel becomes possiible after some space statation explodes in Earth orbit, creating "time strings". Each time string creates a gate between two times and places. Some gates are stable, in that they open and close regularlly between the two times; while others are unstable in that they open or close once or a few times or at irregular intervals. I find it overly convenient that all to many time gates open and close in the one location the books time station is set, that being in an excavated cavern under the Himilayas in 1910. Also, only up timers from the late 20th century find and use the gates on a normal basis. I'd expect down timers to have found gates and used them to explore time. And I'd expect them to have fought the uptimers occassionally. An interesting notion for gaming though is that time travelers can not kill, or save from death, anybody who is historically important. Nor can an up-timer be in the same time period more than once. If they overlap, they just vanish apparently. That rule can be used to keep players in line.