So I've successfully limited my feed to a month and working fine :)
New problem - when testing with the atom feed tester it says the feed is invalid.
Apparently the actual feed date is in this format:
<pubDate>Wed, 12 Dec 2012 10:00:00 UTC</pubDate>
To be accepted as a good feed it has to have either just
'UT' or 'GMT' as the end column
The normal wordpress feed has +0000 at the end of it.
Is there a way of correcting this?