title: how calculate difference or compare(is bigger or not) between to data(Jalali calendar)
body: <p>I use two ngb-datepicker in a page, of course <strong>jalali calendar</strong>, and bind to these two model:</p>
  
  <pre><code>dateModelFrom: NgbDateStruct;
  dateModelTo: NgbDateStruct;  
  </code></pre>
  
  <p>after user select dates, i have 2 <strong>jalali date</strong> with ngb date structure:  </p>
  
  <pre><code>dateModelFrom = {day: 1, month: 1, year: 1398}
  dateModelTo = {day: 3, month: 1, year: 1398}  
  </code></pre>
  
  <p>now, I need to calculate difference between two dates, and check if fromDate is less than toDate or not.   </p>
  
  <p>i can use (<a href="https://github.com/alihoseiny/ngx-persian" rel="nofollow noreferrer">https://github.com/alihoseiny/ngx-persian</a>) or (<a href="https://momentjs.com/" rel="nofollow noreferrer">https://momentjs.com/</a>) and convert these two date and then calculate, but this can not be good, I think must be shorter solution. </p>
  
  <p>also i know there is NgbDateNativeAdapter service(<a href="https://ng-bootstrap.github.io/#/components/datepicker/api#NgbDateNativeAdapter" rel="nofollow noreferrer">https://ng-bootstrap.github.io/#/components/datepicker/api#NgbDateNativeAdapter</a>), i try to convert to javascript Date, and then calculate, but output  is same as input:</p>
  
  <pre><code>let toDay:NgbDateStruct = this._persianNgbCalendar.getToday();;
  let _toDay:Date = this._ngbDateNativeAdapter.toModel(toDay);
  </code></pre>