title: Angular 9 how use/import DateAdapter, NativeDateAdapter "Can't resolve '@angular/material/core/datetime'"
body: <p>I am trying to set the correct (locale based) start of the week for date picker.
  So i used <a href="https://stackoverflow.com/questions/45057191/angular-4-material-2-md-datepicker-set-first-day-of-the-week">this</a> and <a href="https://stackoverflow.com/questions/45019317/matdatepicker-start-week-on-monday/45020144">that</a> question and implemented my own DateAdapter:</p>
  <pre><code>import { NativeDateAdapter } from &quot;@angular/material/core/datetime&quot;;
  import { Injectable } from &quot;@angular/core&quot;;
  import { LOCALE_ID, Inject } from &quot;@angular/core&quot;;
  import { Platform } from &quot;@angular/cdk/platform&quot;;
  import { getLocaleFirstDayOfWeek } from &quot;@angular/common&quot;;
  
  @Injectable()
  export class LocaleDateAdapter extends NativeDateAdapter {
    constructor(@Inject(LOCALE_ID) public locale: string) {
      super(locale, new Platform());
    }
  
    getFirstDayOfWeek() {
      return getLocaleFirstDayOfWeek(this.locale);
    }
  }
  </code></pre>
  <p>i also put it into providers accordingly:</p>
  <pre><code>import { LocaleDateAdapter } from './components/shared-components/locale-date-adapter';
  import { DateAdapter } from '@angular/material/core/datetime';
  ...
  providers: [
  ... 
      {
       provide: DateAdapter,
       useClass: LocaleDateAdapter
      }
  ...
  </code></pre>
  <p>While VS Code seems to be fine with it the compiler moans about:</p>
  <pre><code>ERROR in ./src/app/app.module.ts
  Module not found: Error: Can't resolve '@angular/material/core/datetime' in '...\src\app'
  ERROR in ./src/app/components/shared-components/locale-date-adapter.ts
  Module not found: Error: Can't resolve '@angular/material/core/datetime' in '...\src\app\components\shared-components'
  </code></pre>
  <p>So how to make this work. (I also would be fine with less code to get the start of the week locale aware)</p>