title: How to select rows with the latest time for each date within the last 30 days from now
body: <p>Imagine a table with field '<strong>datetime</strong>'. Example rows:</p>
  <ul>
  <li><strong>2017-01-27 13:06:02</strong></li>
  <li>2017-01-27 05:13:14</li>
  <li><strong>2017-01-23 22:13:56</strong></li>
  <li><strong>2017-01-26 14:02:09</strong></li>
  <li>2017-01-23 13:26:12</li>
  <li>...</li>
  </ul>
  <p>I need to get * from the bold lines, BUT WITHIN the last 30 days from now...</p>
  <p>In other words, rows with the max date in the last 30 days.</p>
  <p>30 rows in total in each case, assuming every day has at least one row...</p>