I am using `ion-datetime` for the month and year dropdown values. When I put `MMMM` in `displayFormat`, it doesn't change the model value. However, if I put the `YYYY` in `displayFormat`, it works perfectly fine. Following is my HTML code:

```
<ion-item>
   <ion-label>Month</ion-label>
   <ion-datetime displayFormat="MMMM" [(ngModel)]="filter_data.month"></ion-datetime>
</ion-item>
<ion-item>
   <ion-label>Year</ion-label>
   <ion-datetime displayFormat="YYYY" [(ngModel)]="filter_data.year"></ion-datetime>
</ion-item>

```

What I am doing wrong? Any help would be appreciated.