Steps to reproduce the issue

(from the Article Manager)

1. Open an article (new or existing)
2. Change the "Start Publishing" date/time via the calendar tool (call this DATE 1)
3. Change the "Start Publishing" date/time manually (via typing in the text field) (call this DATE 2)
4. Save the article. You'll notice DATE 1 is saved; DATE 2 is lost.

Alternatively, instead of saving the article (step 4), you can simply open the calendar tool again, and you'll see the DATE 2 revert to DATE 1.

Expected result

DATE 2 should be saved

Actual result

DATE 1 is saved

System information (as much as possible)

Confirmed in Chrome, Version 59.0.3071.115 (Official Build) (64-bit)
and Firefox 54.0 (32-bit)

Additional comments

You can open and close the calendar tool without losing the ability to manually update the date. But once you make a change to the date or time via the calendar tool, manually changing the date will not stick. This happens whether or not the calendar tool is open when you manually change the date. The issue starts the moment a change to the date/time is made via the tool.