Nouislider date, Angular

import { Component } from '@angular/core';
import { NouiFormatter } from 'ng2-nouislider';
import { BehaviorSubject, Observable } from 'rxjs';
import * as moment from 'moment';

@Component({
  selector: 'app-root',
  templateUrl: './app.component.html',
  styleUrls: ['./app.component.scss'],
})
export class AppComponent {
  someRange = [-12, 0];
  start$ = new BehaviorSubject<number>(-12);
  end$ = new BehaviorSubject<number>(0);
  config: any = {
    tooltips: [new StartFormat(this.start$), new EndFormat(this.end$)],
  };

  onChange(event: [number, number]) {
    this.start$.next(event[0]);
    this.end$.next(event[1]);
  }
}

class StartFormat implements NouiFormatter {
  private start: number;
  constructor(private start$: Observable<number>) {
    start$.subscribe(data => (this.start = data));
  }

  from = () => 0;

  to(value: number): string {
    return moment()
      .utc()
      .subtract(Math.abs(this.start), 'months')
      .toISOString();
  }
}

class EndFormat implements NouiFormatter {
  private end: number;
  constructor(private end$: Observable<number>) {
    end$.subscribe(data => (this.end = data));
  }

  from = () => 0;

  to(value: number): string {
    return moment()
      .utc()
      .subtract(Math.abs(this.end), 'months')
      .toISOString();
  }
}