title: Advancing through relative dates using strtotime()
body: <p>I'm trying to use <code>strtotime()</code> to respond to a button click to advance -1 and +1 days (two buttons) relative to the day advanced to on the previous click.  </p>
  
  <p><strong>Example:</strong>  </p>
  
  <ul>
  <li>It's the 10th of the month, I click "-1 day" button, and now the date reads as the 9th.  </li>
  <li>I click the "-1 day" button again and now the readout states the 8th day. </li>
  <li>I click the "+1 day" button and now the readout states it's the 9th.  </li>
  </ul>
  
  <p>I understand the buttons and the displaying the date and using <code>$_GET</code> and PHP to pass info, but how do I get <code>strtotime()</code> to work on the relative date from the last time the time travel script was called?</p>
  
  <p>My work so far has let me show yesterday and today relative to <em>now</em> but not relative to, for example, the <em>day before yesterday</em>, or the <em>day after tomorrow</em>.  Or if I use my <strong>"last monday"</strong> button, the day before or after whatever that day is.</p>