Add archive as a category

I want to create an archive of my blog month by month but my blog is retrospective. The archive widget is categorising the posts in real time but I would like to categorize my posts as a monthly archive but allocate a different timeline. For example, I’ve uploaded posts since April but would like to assign those posts back to October, then ones to November and so on so readers can zip back to the beginning of my blog easily.
I have one categories drop down and I don’t want it to go there. Everytime I try to create a new category it just puts it all in there so I have 2 category drop downs with exactly the same in each one but different titles. At the moment, readers have to go all the way back to the beginning using a clumsy side navigation.
Is this fixable? http://www.peggotty.net