CodePen Embed - Dropdown Month Picker

<div class="button-container">
  <div class="button-left">
    &#9668;
  </div>
  <div class="button">
    December, 2012
  </div>
  <div class="button-right">
    &#9668;
  </div>
</div>
<div class="year-container">
  <div class="arrow">
  </div>
  <div class="year">
    <div class="year-row">
      <div class="month positive">
        <div class="month-result">
        </div>
        <span>JAN</span>
        <span>2012</span>
      </div>
      <div class="month positive">
        <div class="month-result">
        </div>
        <span>FEB</span>
        <span>2012</span>
      </div>
      <div class="month negative">
        <div class="month-result">
        </div>
        <span>MAR</span>
        <span>2012</span>
      </div>
      <div class="month positive">
        <div class="month-result">
        </div>
        <span>APR</span>
        <span>2012</span>
      </div>
      <div class="month positive">
        <div class="month-result">
        </div>
        <span>MAY</span>
        <span>2012</span>
      </div>
      <div class="month positive">
        <div class="month-result">
        </div>
        <span>JUN</span>
        <span>2012</span>
      </div>
      <div class="month positive">
        <div class="month-result">
        </div>
        <span>JUL</span>
        <span>2012</span>
      </div>
      <div class="month negative">
        <div class="month-result">
        </div>
        <span>AUG</span>
        <span>2012</span>
      </div>
      <div class="month positive">
        <div class="month-result">
        </div>
        <span>SEP</span>
        <span>2012</span>
      </div>
      <div class="month positive">
        <div class="month-result">
        </div>
        <span>OCT</span>
        <span>2012</span>
      </div>
      <div class="month negative">
        <div class="month-result">
        </div>
        <span>NOV</span>
        <span>2012</span>
      </div>
      <div class="month active positive">
        <div class="month-result">
        </div>
        <span>DEC</span>
        <span>2012</span>
      </div>
      <div class="the-year">
        2012
      </div>
    </div><!-- End year row -->
       <div class="year-row">
      <div class="month nodata">
        <div class="month-result">
        </div>
        <span>JAN</span>
        <span>2011</span>
      </div>
      <div class="month nodata">
        <div class="month-result">
        </div>
        <span>FEB</span>
        <span>2011</span>
      </div>
      <div class="month positive">
        <div class="month-result">
        </div>
        <span>MAR</span>
        <span>2011</span>
      </div>
      <div class="month positive">
        <div class="month-result">
        </div>
        <span>APR</span>
        <span>2011</span>
      </div>
      <div class="month positive">
        <div class="month-result">
        </div>
        <span>MAY</span>
        <span>2011</span>
      </div>
      <div class="month positive">
        <div class="month-result">
        </div>
        <span>JUN</span>
        <span>2011</span>
      </div>
      <div class="month negative">
        <div class="month-result">
        </div>
        <span>JUL</span>
        <span>2011</span>
      </div>
      <div class="month positive">
        <div class="month-result">
        </div>
        <span>AUG</span>
        <span>2011</span>
      </div>
      <div class="month positive">
        <div class="month-result">
        </div>
        <span>SEP</span>
        <span>2011</span>
      </div>
      <div class="month positive">
        <div class="month-result">
        </div>
        <span>OCT</span>
        <span>2011</span>
      </div>
      <div class="month positive">
        <div class="month-result">
        </div>
        <span>NOV</span>
        <span>2011</span>
      </div>
      <div class="month positive">
        <div class="month-result">
        </div>
        <span>DEC</span>
        <span>2011</span>
      </div>
      <div class="the-year">
        2011
      </div>
    </div>
  </div>
</div>

body {
  background: #F6F2EB;
  line-height: 1em;
}

.box(@prop: border-box) {
  box-sizing: @prop;
  -moz-box-sizing: @prop;
  -webkit-box-sizing: @prop;
}

div.button-container {
  zindex: 1;
  background: -webkit-linear-gradient(top, white, #E6E6E6);
  background: -moz-linear-gradient(top, white, #E6E6E6);
  width: 250px;
  height: 35px;
  margin: 0 auto;
  margin-top: 50px;
  padding: 0;
  border: 1px solid rgba(0,0,0,0.30);
  border-top: 1px solid rgba(0,0,0,0.2);
  border-radius: 4px;
  -moz-border-radius: 4px;
  box-shadow: 0px 1px 2px rgba(0,0,0,0.15),
              0px 1px 0px rgba(255,255,255,0.4) inset;
  -moz-box-shadow: 0px 1px 2px rgba(0,0,0,0.15),
                   0px 1px 0px rgba(255,255,255,0.4) inset;
  -webkit-box-shadow: 0px 1px 2px rgba(0,0,0,0.15),
                      0px 1px 0px rgba(255,255,255,0.4) inset;
  .box;
}

div.button-left {
  float: left;
  font-size: 13px;
  padding: 7px;
  height: 100%;
  width: 15%;
  text-align: center;
  vertical-align: center;
  color: rgba(0,0,0,0.7);
  .box;
}

div.button-right {
  float: left;
  font-size: 13px;
  height: 100%;
  width: 15%;
  padding: 7px;
  text-align: center;
  vertical-align: center;
  color: rgba(0,0,0,0.7);
  transform: rotate(180deg);
  -webkit-transform: rotate(180deg);
  -moz-transform: rotate(180deg);
  .box;
}

div.button-left:hover, div.button-right:hover {
  background: rgba(0,0,0,0.03);
  cursor: pointer;
}

div.button {
  float: left;
  width: 70%;
  text-align: center;
  text-shadow: 0px 1px 0px rgba(255,255,255,0.2);
  .box;
  padding: 9px 0;
  font-size: 13px;
  font-family: Arial, sans-serif;
  border-left: 1px solid rgba(0,0,0,0.15);
  border-right: 1px solid rgba(0,0,0,0.15);
  color: rgba(0,0,0,0.65);
  cursor: pointer;
  box-shadow: -1px 0px 0px rgba(255,255,255,0.4),
               -1px 0px 0px rgba(255,255,255,0.4) inset;
}

div.button:hover {
  background: rgba(0,0,0,0.03);
}

div.pressed {
  background: rgba(0,0,0,0.03);
  box-shadow: inset 1px 0 4px rgba(0,0,0,0.07),
              inset -1px 0 4px rgba(0,0,0,0.07);
}

div.year-container {
  width: 700px;
  margin: 10px auto;
  display: none;
}

div.arrow {
  background: #FFF;
  height: 11px;
  width: 10px;
  border-left: 1px solid rgba(0,0,0,0.50);
  border-top: 1px solid rgba(0,0,0,0.50);
  margin: 0 auto;
  transform: rotate(45deg);
  -webkit-transform: rotate(45deg);
  -moz-transform: rotate(45deg);
  zindex: 2;
  border-bottom-right-radius: 5px;
}

div.year {
  border: 1px solid rgba(0,0,0,0.3);
  background: #FFF;
  border-radius: 4px;
  -moz-border-radius: 4px;
  -webkit-border-radius: 4px;
  box-shadow: 0px 0px 1px rgba(0,0,0,0.22);
  width: 100%;
  margin: -6px 0 0 0;
  padding: 5px;
  .box;
}

div.year-row {
  height: 40px;
  padding: 5px 0;
  border-bottom: 1px dotted #DDD;
}

div.year-row:first-of-type {
  margin-top: 0;
  padding-top: 0;
}

div.year-row:last-of-type {
  margin-bottom: 0;
  padding-bottom: 0;
  border-bottom: none;
}

div.month {
  float: left;
  .box;
  height: 40px;
  width: 7%;
  border-radius: 2px;
  -moz-border-radius: 2px;
  -webkit-border-radius: 2px;
  border: 1px solid rgba(0,0,0,0.15);
  padding: 2px;
  margin-right: 5px;
  opacity: 0.35;
}

div.month:hover {
  opacity: 1.0;
  cursor: pointer;
}

div.nodata:hover {
  cursor: auto;
  opacity: 0.35;
}

div.positive:hover span:last-of-type, div.negative:hover span:last-of-type {
  opacity: 1.0;
}

div.positive:hover div.month-result, div.negative:hover div.month-result {
  width: 100%;
}

div.month span {
  font-size: 9px;
  color: #222;
  display: inline-block;
  font-family: Arial, sans-serif;
  text-indent: 1px;
  width: 100%;
  line-height: 10px;
}

div.month span:last-of-type {
  float: left;
  color: #BBB;
  opacity: 0;
  margin: 0;
}

div.month-result {
  .box;
  border-radius: 1px;
  -moz-border-radius: 1px;
  -webkit-border-radius: 1px;
  border-bottom-left-radius: 0;
  border-bottom-right-radius: 0;
  -webkit-border-bottom-left-radius: 0;
  -webkit-border-bottom-right-radius: 0;
  -moz-border-bottom-left-radius: 0;
  -moz-border-bottom-right-radius: 0;
  width: 50%;
  height: 5px;
  margin-bottom: -3px;
}

div.positive div.month-result {
  background: #468847;
}

div.negative div.month-result {
  background: #B94A48;
}

div.nodata div.month-result {
  background: #CCC;
}

div.active {
  opacity: 1.0;
}

div.active div.month-result {
  width: 100%;
}

div.the-year {
  background: #EEE;
  float: left;
  .box;
  width: 7%;
  height: 40px;
  text-align: center;
  padding: 12px 0;
  vertical-align: center;
  font-family: Arial, sans-serif;
  font-size: 11px;
  color: #222;
   border-radius: 2px;
  -moz-border-radius: 2px;
  -webkit-border-radius: 2px;
  border: 1px solid rgba(0,0,0,0.1);
}

View Compiled

$('.button').click(function() {
  $(this).toggleClass('pressed');
  $('div.year-container').toggle();
});
  
$('.month').click(function() {
  $('.month').removeClass('active');
  $(this).addClass('active');
});

Run Pen

This Pen is owned by Travis Loncar on CodePen.

See more by @tbloncar on CodePen

External CSS

This Pen doesn't use any external CSS resources.

External JavaScript

//cdnjs.cloudflare.com/ajax/libs/jquery/2.1.3/jquery.min.js