When scheduling a post to publish, not possible to change and publish immediately

[ericaherschorn]

Describe the bug
During testing, I noticed that I was unable to publish a post after scheduling a publish in the future. If you convert your post into a classic editor post the functionality works fine, leading me to believe it is a Gutenberg bug.

To Reproduce
Steps to reproduce the behavior:

1. Go to "create a new post" or "edit a post"
2. Change the date of publish to the future, and click "schedule" (successfully schedules for the future)
3. Go back to edit the post, and change the publish date to today, or a date in the past, and click "publish"
4. Error occurs - it takes you back to the schedule post menu, and there is no way to save the new date.

Expected behavior
Post publishes

Screenshots
Future date - schedule button shows

Change date to past - publish button shows

Click "publish, and this menu slides in from the right, showing a date in the past (today is Aug 15)

Desktop (please complete the following information):

• OS: MacOS High Sierra v. 10.13.5
• Browser: Chrome
• Version 68.0.3440.106

Smartphone (please complete the following information):
Not tested on device.

Additional context

• WordPress Version 4.9.8
• Gutenberg Version 3.4.0

[boemedia]

I tested this with Gutenberg 3.8.0 and WP 4.9.8. I can confirm going straight from schedule to publish doesn't work. Apparently, you have to go back to concept mode now before you're able to publish immediately. I love how Hootsuite handles this: the 'schedule button' and 'send now' are detached. In my opinion, this gives a clearer UX.

Hootsuite publishing screen, with a calendar icon if you want to schedule a post.

When you click that calendar icon, the pane expands with scheduling options.

[oandregal]

Consolidating in #10930