At all times, in all history books, in everything that we can remember we have read and ourselves reproduced timelines horizontally. Up until a few years ago, when PowerPoint made its entry, and annoyingly enough, lacked a template for timelines was when we simply started to create vertical timelines.
This is completely wrong according to your own perception but above all totally wrong for your audience. As a presenter you have to eliminate as many barriers as possible for your recipient, the fewer obstacles the easier to absorb your message.
Below, I have exemplified both a modern vertical timeline (which really only exists because of ignorance or sheer laziness), and a horizontal timeline so that a timeline can be reproduced. Feel free to download the presentation and test it.
Link to the example(PDF)
Link to the example(PPT)