Entity dates don't save properly

[royvanos]

Preconditions

1. Magento 2.1.8
2. MySQL 5.6.37

Steps to reproduce

1. Any Magento 2.1.8 installation
2. Admin interface language set to nl_NL (Dutch)
3. Create a product with any special date range and save it, the date can't be the same as the current day

Expected result

1. Product special price range should still be the same

Actual result

1. Product special price dates show the current date

There also is a javascript error whenever a date object tries to be instanciated: Error creating Date object from string.

After a bit of debugging it looks like the jquery-ui-timepicker-addon.js tries to instanciate a date object using the date format that belongs to the set adminhtml locale (DD-MM-YYYY in my case), it seems like the addon can't handle any date formats other than YYYY-MM-DD (which is understandable) but Magento injects a date using the corresponding date format for the set adminhtml locale, setting the adminhtml locale to en_US works fine.

This seems to happen to every date field there is.

[orlangur]

Duplicate of #10580 and a bunch of other issues.