title: How to convert string (time or date time) in timestamp and save in mongodb
body: <p>I am accepting time as string from user like "10:00am" or if needed i can accept "2015-08-27 10:00am" from user. </p>
  
  <p>Need to use rails function <em>Date("2015-08-27 10:00am")</em> to convert string in date and then needs to save in db?</p>
  
  <p>I'm currently using</p>
  
  <ul>
  <li>ruby 2.2.2</li>
  <li>rails 4.1.4</li>
  <li>mongoid 4.0.0</li>
  </ul>