Is there a unified platform that lists conference/journal deadlines? Is there any unified platform that would list the deadlines (especially the submission deadlines) for conferences/journals? Ideally I would like to select a few conferences/journals I am interested in publishing to, and the website would give me the list of upcoming deadlines. 

Right now I have to check out each individual conference/journal website, which is pretty tedious and error-prone (all the more so as it is pretty common for deadline to be extended).

I am mostly interested in the field of computer science > machine learning, and English-speaking venues.
WikiCFP offers this. To use this feature, you are required to register and sign in to your account.

Once you are logged in, find the Calls for Papers (CFPs) you are interested in and choose "Add to my list". (You can add multiple CFPs at a time if they appear in the same search results or "recent CFPs" list).

Then, from the sidebar, choose "My list" or "Timeline" to see all the deadlines for the conferences you have selected in your preferred format. Timeline view is pretty nice; it shows you the registration, submission, notification, and camera-ready deadlines, as well as the dates of the actual conference. Here is a screenshot:



However, WifiCFP relies on user-submitted content (for both original CFP submissions, and updates e.g., for deadline extensions). So the usefulness of this for you will depend on how active people in your field are on WikiCFP.