title: How I can create Angular custom Date Pipe
body: <p>I'm working on angular 5 I want to create a custom date pipe that allows me to subtract some days from a date  :</p>
  <p>This how I display my date value :</p>
  <pre><code> &lt;span&gt;{{data.nextCertificateUpdate | date:'yyyy-MM-dd'}}&lt;/span&gt;  
  </code></pre>
  <p>for example this display a value like  : <strong>2018-08-29</strong></p>
  <p>I ask if it's possible to create a pipe that allow me to substract a number of days for example 28 from this date ?</p>
  <p>Something like :</p>
  <pre><code>&lt;span&gt;{{data.nextCertificateUpdate | mypipe:28 }}&lt;/span&gt;  
  </code></pre>
  <p>and this should display value like : <strong>2018-08-01</strong></p>
  <p>Thanks for any help</p>