Year 0000 marked as valid in Datepicker

[dzagaduroska]

Bug description:
In the popup Datepicker a date string in which the year is 0000 is marked as valid. The value of the ngModel is null, but the status is "VALID" which is confusing. In my opinion these dates should be marked as "INVALID".
E.g 0000-04-09

Link to plunker: http://plnkr.co/edit/4dTDk1?p=preview

Version of Angular, ng-bootstrap, and Bootstrap:

Angular: 6.1.3

ng-bootstrap: 3.2.0

Bootstrap: 4.0.0-alpha.6

[pkozlowski-opensource]

In the popup Datepicker a date string in which the year is 0000 is marked as valid. The value of the ngModel is null, but the status is "VALID" which is confusing.

We should investigate this. If the model is null than it should only denote an empty value (and in this case a control can be valid if it is not marked as required). But yes, we should be marking control as invalid if 0000 is entered.

In my opinion these dates should be marked as "INVALID". E.g 0000-04-09

True, we should probably exclude year 0 in the Gregorian calendar: https://en.wikipedia.org/wiki/Year_zero

[dzagaduroska]

Thanks :)