title: Open an HTML select tag after one option has been selected
body: <p>I have an HTML select tag like this:</p>
  
  <pre><code>&lt;select name="since_date"&gt;
      &lt;option value="&lt;?php $sixmonths = date('Y-m-d', strtotime('-6 months')); echo $sixmonths; ?&gt;"&gt;6 months&lt;/option&gt;
      &lt;option value="&lt;?php $fivemonths = date('Y-m-d', strtotime('-5 months')); echo $fivemonths; ?&gt;"&gt;5 months&lt;/option&gt;
      &lt;option value="&lt;?php $fourmonths = date('Y-m-d', strtotime('-4 months')); echo $fourmonths; ?&gt;"&gt;4 months&lt;/option&gt;
      &lt;option value="&lt;?php $threemonths = date('Y-m-d', strtotime('-3 months')); echo $threemonths; ?&gt;"&gt;3 months&lt;/option&gt;
      &lt;option value="&lt;?php $twomonths = date('Y-m-d', strtotime('-2 months')); echo $twomonths; ?&gt;"&gt;2 months&lt;/option&gt;
      &lt;option value="&lt;?php $onemonth = date('Y-m-d', strtotime('-1 month')); echo $onemonth; ?&gt;"&gt;1 month&lt;/option&gt;
  &lt;/select&gt;
  </code></pre>
  
  <p>And I want when for example 4 months is selected a new select tag to appear like this one:</p>
  
  <pre><code>&lt;select name="until_date_4months"&gt;
      &lt;option value="&lt;?php $threemonths = date('Y-m-d', strtotime('-3 months')); echo $threemonths; ?&gt;"&gt;3 months&lt;/option&gt;
      &lt;option value="&lt;?php $twomonths = date('Y-m-d', strtotime('-2 months')); echo $twomonths; ?&gt;"&gt;2 months&lt;/option&gt;
      &lt;option value="&lt;?php $onemonth = date('Y-m-d', strtotime('-1 month')); echo $onemonth; ?&gt;"&gt;1 month&lt;/option&gt;
      &lt;option value="&lt;?php $now = date('Y-m-d'); echo $now; ?&gt;"&gt;Now&lt;/option&gt;
  &lt;/select&gt;
  </code></pre>
  
  <p>Every time a period is chosen (Ex. 6 months, 5 months) a new select must appear with options lower than the selected one plus a new option "Now" like above. How is this possible? What is the approach? I want to use these information with a submit button. (Send with JQuery a GET request to a specific PHP file &lt;- this I know how to handle). 
  I can for example use JavaScript like this:</p>
  
  <pre><code>function sinceDateValue(selection) {
      if (selection.value == "&lt;?php $fourmonths = date('Y-m-d', strtotime('-4 months')); echo $fourmonths; ?&gt;") {
          document.getElementbyId(until_date_4months).style.display = "block"
      }
  }
  </code></pre>
  
  <p>and use for the second submit <code>id="until_date_4months"</code> and <code>style="display: none;"</code>. Also for the first submit <code>onchange="testValue(this);</code> but this will just show the submit and I want to send only two information with the form: since_date and until_date...</p>