Steps to reproduce the issue

1. Open the Article Edit screen, click the Publishing tab
2. Click the Start Publishing field 'open calendar' button
3. With the calendar open, change any of the datetime values (e.g. year, month, day, hour etc.) manually by typing
4. Click away / blur the field
5. Click Save

Expected result

Typed in value is retained

Actual result

Calendar datetime reverts to last clicked / picked value rather than typed in value

System information (as much as possible)

Joomla 3.7.5
PHP 5.6
Apache 2.4

Additional comments

See the issue in action at http://support-media.muckypuddle.me/mAJf

Got a fix for this in the works, but found it as part of #16084 and wanted to log separately.