How to use the datetime picker with Firebase

I’m trying to add a date selecting function into my app but when I click the “add” button, the date doesn’t get saved into my Firebase database.

Here is the code html code:

          <ion-datetime displayFormat="DD MMM" [(ngModel)] = "">

My item.model.ts code

export interface Item{
    key?: string;
    date?: string;

And my ts code

import { Component } from '@angular/core';
import { IonicPage, NavController, NavParams } from 'ionic-angular';
import { Item } from '../../models/item/item.model';
import { DutiesList } from '../../services/duties/';
import { Observable } from 'rxjs/Observable';
import { ToastService } from '../../services/toast/toast.service';

  selector: 'page-add',
  templateUrl: 'add.html',
export class AddPage {
  private myDate: string;
  viewDuty$: Observable<Item[]>
  item: Item = {
    date: '',
  constructor(public navCtrl: NavController, 
    public navParams: NavParams, 
    private addDuty: DutiesList,
    private toast: ToastService) {}

  ionViewDidLoad() {
    console.log('ionViewDidLoad AddPage');

    addItem(item: Item){
            this.addDuty.addItem(item).then(ref =>{
'Duty added!');

the ion-datetime control seems to server string type. firebase needs a firebase timestamp type.

Anyone there how to get this work proper?