Hotfix: Leap year should throw exception.

[Johannestegner]

Type of change

• [ ] New feature
• [X] Bug fix
• [X] Security patch
• [ ] Documentation update

Description

Adds a test for leap-year check.
Changed date parsing implementation in date check to LocalDate. Updated error message for errors that have to do with dates.

Motivation

Fixes a potential vulnerability where valid would return true on dates which are not valid (out of range).

Checklist

• [X] I have read the CONTRIBUTING document.
• [X] I have read and accepted the Code of conduct
• [X] Tests passes.
• [X] Style lints passes.
• [X] New tests added which covers the added code.