date.isValid is not a function

When I use the angular4 to build a website, I need to use a ng-bootstrap component as a datepicker, but I got a problem: and here is my code: what is wrong with my code!

import { Component, OnInit } from '@angular/core';
import { JhiLanguageService } from 'ng-jhipster';
import { Principal, AccountService, JhiLanguageHelper } from 'app/core';
import { SettingsService } from 'app/account/settings/settings.service';
import { HttpErrorResponse, HttpResponse } from '@angular/common/http';
import {NgbDateStruct, NgbDateAdapter, NgbDate, NgbCalendar} from '@ng-bootstrap/ng-bootstrap';
import { Moment } from "moment";
import { User} from "app/core";

@Component({
    selector: 'jhi-settings',
    templateUrl: './settings.component.html',
    styleUrls: ['settings.css']
})
export class SettingsComponent implements OnInit {
    error: string;
    success: string;
    settingsAccount: any;
    languages: any[];
    genders: [];
    levels: [];
    avatarFileName: string;
    avatarFile: File;

    mydob= {year: 2018, month: 9, day: 15};

    constructor(
        private account: AccountService,
        private principal: Principal,
        private languageService: JhiLanguageService,
        private languageHelper: JhiLanguageHelper,
        private settingService: SettingsService,
        private today : NgbCalendar
    ) { }
}

Date of birth