In contrast, a time-travel novel involves the protagonist travelling to another time, due to his or her desire to do so.
A Time-Slip novel whether futuristic, contemporary or historical, has by its nature, two timelines. These two threads must mesh throughout the novel but each can have its own tense, voice and storyline.
For example, in a historical time-slip you could write the present thread in first person and the historical thread in third or vice versa. You may also want to lead readers into the historical section by altering the voice and/or using historical language idioms. You may want to alter the gender of the main character or set up your format in a different way. For example, the past thread could be in a letter or journal. In a futuristic time, you could use machines, robotic voices or alien concepts to differentiate one thread from another. This all helps to make the setting clear and cements the timeline in the reader’s mind.
Your novel could start with the present thread or you could write a prologue that captures a pivotal scene in the future or past story that then is used as a flashback or flashforward point to which your story progresses.
Each story must impact the other eventually, otherwise there’s no conflict and you may as well write two separate stories. Usually there is a mystery or conflict that affects both main characters and you want to exploit this often by changing from one thread to the other fairly frequently, say two chapters of present then two or three of past/future. There’s no need to make them half and half but you don’t want to stay in one thread so long that the reader forgets the other story.
Which brings us to the transition between one story and the next. This must be done as seamlessly as possible so that the readers aren’t jerked out of one timeline into the other. Also, one story thread may appeal to a reader more than the other. You are asking them to read two stories that may not connect until the end and so you need to make each thread interesting and engaging on it’s own. If one thread doesn’t have as much “story” in it, then avoid filling it with fluff and concentrate on the thread’s placement within your story. You can hide its shorter length if you pace it out judiciously and ensure that each scene moves the story along in a gripping way.
Be aware that if your character travels to another time, she/he may be dressed inappropriately, won’t understand the culture necessarily and will obviously be not of that time, unless you somehow disguise the character’s origins. This can of course, be used to advantage or create conflicts that form the core of your story.
Turning points, plot points and black moments are all important in each story and may not fall into place together. Juggling the timelines can get tricky and it would be wise to do an outline of how your story will work overall, noting how one plot point could trigger a change in the other thread and when to introduce the transition. As your novel heads towards the end, you will want to tie up the timelines and finish one before resolving the other.
Time-slips can be challenging but they can give you scope for further characterisation and deeper plot. And they can be enormous fun.