Steps to reproduce the issue

Activate plugin "User - Profile"
Open an existing user (also with new users)
Add a "Date of birth" with date picker --> value will be i.e. 2017-05-10 (English) or i.e. 10.05.2017 (German)
Save User

Expected result

Successfully save User with Date of Birth

Actual result

Error: "Save failed with the following error: The date of birth you entered is invalid. Please enter a valid date."

System information (as much as possible)

Joomla 3.7.0
PHP 5.6.21 or 7.0.14 (tested on both)
DB Version 5.5.5-10.1.13-MariaDB or 5.5.52-nmm1-log

Additional comments

Tested with new and existing users in Frontend and Backend
Always same error.