How to set displayFormat dynamically for localization?


I am using ionic 2 and would like to set a date format value to displayFormat attribute of ion-datetime dynamically as this application can let the user to choose the format of date.

This is part of edit.ts'date_format').then((date_format) => {
      this.date_format = date_format;

Below is part of edit.html, but the format is Dec 12, 2015, even though date_format is “YYYY/MM/DD”.

<ion-datetime [displayFormat]="date_format" formControlName="base_date"></ion-datetime>
<p>{{ date_format }}</p>   <- this shows YYYY/MM/DD

Tried below, but the format is still Dec 12, 2015.

<ion-datetime displayFormat="{{ date_format }}" formControlName="base_date"></ion-datetime>

How to set the value for displayFormat dynamically?

Thanks in advance