Facebook uses an internal way of scheduling (for pages) or backdating (for personal profiles or pages) status updates or photos. Twitter can schedule posts using TweetDeck (which can be used in the browser, no software required to be installed).
But what about Google Plus?
You have two options when you want to schedule stuff in G+.
Method 1: HootSuite
This only works for G+ pages and not personal profiles, so if you operate one or more G+ pages, you can schedule your posts using HootSuite. Works in-browser and no software is required to use it.
When scheduling things in HootSuite, you do not have to have the site opened in order for scheduled posts to publish. You login, schedule it, logout, and it will post on time (similar to TweetDeck which also does not require you to be logged in for scheduled posts to publish on the date/time you set).
Method 2: Do Share for Google Chrome
This one does allow you to schedule posts on your personal G+ profile, but with a catch. The catch is that in order for Do Share to work, Chrome has to be running. This means you can’t just schedule something and then close the browser, because then the scheduled post won’t be posted.
Do Share is best described as a “resident add-on” where it requires Chrome to work and will not work independently of the browser. You can consider this the same as Mozilla Thunderbird’s Send Later add-on, where in order for that to work, Thunderbird has to be running.
Being that many leave their web browsers open all the time anyway, this shouldn’t be a problem for most people.
While true that you don’t have “power user” scheduler options like Twitter and Facebook have, the above two options for G+ are better than nothing.