title: Update Hour (not date) in MongoDB Date Object
body: <p>Is it possible to update the hour for a date entry on an mongo document without recalculating the entire date?</p>
  
  <p>Here is my schema:</p>
  
  <pre><code>book = {
      name: String,
      release: Date
  }
  </code></pre>
  
  <p>Here are some documents:</p>
  
  <pre><code>{name:"book1", release: ISODate("2016-05-18T16:00:00Z")}
  {name:"book1", release: ISODate("2016-05-17T17:00:00Z")}
  {name:"book1", release: ISODate("2016-03-12T01:00:00Z")}
  </code></pre>
  
  <p>and so on.</p>
  
  <p>As you can see, the hours for the books are 16:00, 17:00, and 01:00.</p>
  
  <p><strong>I would like to set all the hours to 00:00 while keeping the actual day, month and year unchanged</strong></p>
  
  <p>Is that possible? </p>