title: Why javascript set month gonna wrong month?
body: <p>I found it when i set month</p>
  
  <ol>
  <li>Today is <code>08-31</code></li>
  <li>If i increase month, then today will be <code>09-30</code></li>
  <li>But it return <code>10-01</code></li>
  <li>If i decrease month, then it returns <code>09-01</code></li>
  </ol>
  
  <p><div class="snippet" data-lang="js" data-hide="false" data-console="true" data-babel="false">
  <div class="snippet-code">
  <pre class="snippet-code-js lang-js prettyprint-override"><code>date = new Date()
  console.log( date )
  // Fri Aug 31 2018 11: 28: 47 GMT + 0900(한국 표준시)
  
  console.log( date.getMonth() ) 
  // 7
  
  console.log( date.setMonth(date.getMonth() + 1) )
  // 1538360927857
  
  console.log( date.toISOString() )
  // "2018-10-01T02:28:47.857Z"
  
  console.log( date.setMonth(date.getMonth() - 1) )
  //1535768927857
  
  console.log( date.toISOString() )
  //"2018-09-01T02:28:47.857Z"</code></pre>
  </div>
  </div>
  </p>
  
  <p>Can anyone explain me why it happens?</p>