title: How to calculate Customer since i.e Duration of customer in Typescript
body: <p>I want to find out how long a person has been a customer.
  I mean I simply want to subtract current date from start date.
  However I can't understand what I am doing wrong.</p>
  
  <pre><code>customerStartDate: String;
  currentDate: any = '';
  this.customerStartDate = this.sampleData1.customerStartDate;    
  this.currentDate = new Date();
  // Get Customer Age
  this.customerAge = this.currentDate.getTime() - this.customerStartDate.getTime();
  </code></pre>
  
  <p>However current age is giving error.
  How can I calculate the Customer Duration in typescript.</p>