title: In Moment.js, why doesn't subtract by 'week' work?
body: <p>I'm using <code>Moment.js</code> and following line of code doesn't seem to actually do anything:</p>
  
  <pre><code>moment().subtract('week', 1)
  </code></pre>
  
  <p>In the documentation, it shows an example of subtract being used with <code>'days'</code> but it doesn't specify that it's the only string you can use. In most of the other functions, it's an option where you can use <code>'days'</code>, <code>'weeks'</code>, <code>'months'</code>, so I'm not sure if I'm doing something wrong or <code>'weeks'</code> just isn't supported:
  <a href="http://momentjs.com/docs/#/manipulating/subtract/" rel="noreferrer">Moment.js Subtract Documentation</a> </p>
  
  <p><strong>Here's the example of subtracting days:</strong> </p>
  
  <pre><code>moment().subtract('days', 7);
  </code></pre>
  
  <p>It's also what I ended up using instead of <code>'weeks'</code> but I'm still curious why <code>'weeks'</code> aren't supported. </p>