This snippet displays every post by postDate, which lists a bunch of “October 2015”. I am trying to filter by month/year, so only one “October 2015” shows then one “November 2015”, then one “December 2015” so on and so forth. So choosing the month, shows all posts from that month.
<div class="form-field select">
<label for="blog-date">by date</label>
<div class="custom-select">
<select name="blog-date" id="blog-date" onchange="window.location.href=this.value;">
<option value="/blog">All Time</option>
{% cache %}
{% nav entry in craft.entries({ group: 'blog', order: 'postDate desc', limit: null }) %}
<option value="{{ entry.postDate | date('F-Y') }}"
{% if craft.request.getLastSegment() == entry.postDate %}selected{% endif %}>
{{ entry.postDate | title | date('F Y') }}
</option>
{% endnav %}
{% endcache %}
</select>
</div>