Breeze/Angular: date input directive not working/strange behaviour - angularjs

Question

I'm trying to get this directive (https://github.com/eight04/angular-datetime) to work in my project. It works alright in the following plunk:
http://plnkr.co/edit/DawLbi?p=preview
But when I plug in the same code (below) into my larger scale project, it doesn't work and I have no idea why and where to search.
<input type="text" datetime="dd.MM.yyyy" ng-model="vm.firstDate" class="form-control" data-z-validate />
What happens: The input displays the formatted date correctly, but when I click on the year (or some other component) and try to change it using the number keys (enter a new year), it clears out the whole field (upon entering the first digit)! Using the up/down arrow keys works.
The problem happens only when I use the data-model on a breeze-entity. If I just put a simple js Date object on my controller and link to that one, it works.
I also tried extending my breeze entity like so:
function registerRechnungRec(metadataStore) {
metadataStore.registerEntityTypeCtor(g14EntityNames.rechnung, RechnungRec, postCtorInitializer);
function RechnungRec() {
this.datumExt = moment().toDate(); // doesn't work
}
function postCtorInitializer(rechnung) {
// this works, but not tracked by breeze then
rechnung.datumExtDate = moment(rechnung.datum).toDate();
}
// also no luck with that one:
// http://stackoverflow.com/questions/26982624/angular-and-breeze-edit-date-object
//addDateWrapperPropertyExt(RechnungRec, 'datum', 'datumExtDate');
}
A normal input with type date works just fine for entering/selecting the date. But I need to do some momentjs formatting afterwards to save the date correctly to the database then before I call entityManager.save(). Also, I don't really need the whole date picker component, a simple date input is all we need.
So how do I find out what's going wrong where?
Or is there another suggested date input directive/component, or an easier way to just have a text input field that allows me to input a date (no time) or a null date?