Selective substrings in TOC - calibre

Question

I have 100 HTML files with a structure like this:

<div id="message-1">14 April 2015</div>
<div id="message-2">16 April 2015</div>
<div id="message-3">1 June 2015</div>
<div id="message-4">5 June 2015</div>
<div id="message-5">12 November 2015</div>
<div id="message-6">14 November 2015</div>
<div id="message-7">18 November 2015</div>

I use this xpath to create TOC level 1:

//h:div[starts-with(@id, "message-")]

With this xpath I have an entry for every day in the TOC:

14 April 2015
16 April 2015
1 June 2015
5 June 2015
12 November 2015
14 November 2015
18 November 2015

Is it possible to generate first occurrences of each month as Level 1? like this:

April 2015
June 2015
November 2015

or should I modify the original document itself?