Alternate Timelines have been created previously, (a total of 39 time lines) due to the resetting of a time loop set up by Jane Chatwin with the use of a magical Pocket Watch gifted to her by Umber. These were set up as a counter measure for The Beast. Alternate Timelines were created due to Jane Chatwin altering something in a timeline, hence different outcomes. The very same Pocket Watch is speculated as well as the reason that Jane Chatwin was able to go back in time and become the Watcher Woman.
The start of the series is presented as the 40th Timeline. In the book, Jane Chatwin destroys the watch to prevent Quentin Coldwater from undoing their timeline from resetting, after they were successful in defeating The Beast, at the cost of Alice Quinn's life. In the TV series, Jane Chatwin is strangled to death by a possessed Mike McCormick by The Beast while in the "Clean Room". This results in the 40th Timeline being the final one as no one is there to reset it anymore.
Dean Henry Fogg has been noted the only individual to have memories of the Alternate Timelines existence, aside from Jane Chatwin.
In the 23rd timeline Alice Quinn is the sole survivor against the Physical Kids encounter with The Beast. She lives in a post apocalyptic world. Alice is notably more unkempt and tired "broken", with one of her fingers notably disfigured, a price she says was the least she had to pay to learn information about Shades. Alice describes Quentin Coldwater's death as being ripped apart piece by piece. Apart from the physical dismemberment The Beast also ripped apart Quentin's Shade, and everything else about him. Alice Quinn has been noted to have expressed an obsession towards shades in this timeline, to lay Quentin's soul to rest.
The current timeline started at the start of the series. The change made by Jane Chatwin was the admission of Julia Wicker into BrakeBills, in hopes of making her stronger. This resulted in Julia Wicker seeking the Hedge Witches and joining their ranks and eventually joining the Free Trader Beowulf.