title: How to set warnings when time is almost up on DateTime
body: <p>I have due_dates as a column in my db and its a Datetime format.  I want my rails app to be able to detect when there is a day left or an hour left etc. based on the <code>datetime</code> value.  every request will have a due date so each datetime will be unique.  I know how to make notifications I just don't know how to make the timer aspect or however you're suppose to detect these <code>datetimes</code> before or exactly when they pass the date.</p>
  
  <p>ex.<br>
   today &lt;-- 11/5/2016 |(one day)| 11/6/2016 &lt;--duedate     <strong>db update/ send notification "one day left"</strong></p>