So it outputs a
<p>Posted on 31 august 2014</p>?
Not much you can do with that if you are trying to turn it into a ‘calendar like’ icon.
At the very least it should be broken up into separate parts like…
<p class="post-date"> <span class="post-date-head">Posted on</span> <span class="post-date-day">31</span> <span class="post-date-month">August</span> <span class="post-date-year">2014</span> </p>
That way we have separate elements that can be styled individually.
Get your PHP to spit that out…and you have a shot.