It depends. TimelineJS does not work with WordPress.com sites. We are researching ways to address this.
By default, self-hosted WordPress strips `iframe` tags from HTML created by some user roles, but not by others. People should generally install the plugin.
If you are able to install plugins to your Wordpress installation, we have a WordPress plugin for Timeline. It supports embedding Timelines with WordPress "shortcode", and as of version 18.104.22.168, it also has experimental "oembed" support. That means that you can take a direct URL to a timeline (like you get with the "get link to preview" button) and put it in a post on a line by itself, and it should be embedded. (Make sure you've updated the plugin before you try it.)
If you want to see the WordPress plugin in action, it is installed on `knightlab.northwestern.edu`