How can I view OSM as it was a year ago?

If nine months ago is good enough, you could start with an old NZ extract from Geofabrik. If not, someone will probably have an old planet file or an extract that covers it from slightly early earlier.

Once you've got that, you need to render it. The switch2osm "serving tiles" instructions** are probably the best bet if you want to recreate the standard map as of a year ago. Personally, I'd probably go with the "Manually building a tile server (12 04)" flavour as they involve building more locally, and are less likely to fail due to dependency changes since they were written.

If you don't have an physical machine spare to do this on it's fairly easy to do the work on a virtual machine that should fit on the average laptop or desktop (most of England, roughly 4 times the size of NZ in OSM terms, fits in a 2Gb VM OK).

As written those will use the stylesheet as it existed early last year. The newer stylesheet doesn't look that different, but is easier to modify if you want to.

Other rendering options are also available - Maperitive might work for example, but I'm not sure how it would cope with the file size. There's also JOSM of course (though you'll probably need to cut bits out of the extract with osmosis so that they're small enough to load.

** These are now slightly outdated in that they don't use the latest versions of all packages, but if you stick to the versions specified, should work.

An easy but not-that-good way: you could view a map of the fosm.org OSM fork (it was created based on OSM data shortly before 12 September 2012). They seem to use a very similar style sheet making comparisons easy. If there have not been many changes to your area (which could well be the case in many areas – I guess edits are more likely if the area was heavily affected by the redaction) in the fork, you will see OSM as it was in September 2012.

A side note: do not use information from the fork map for editing at OSM (the license does not allow this).

There are also a number of services that don't update their data very often. Cloudmade's maps seem to be still rendered based on data from between approximately May-August 2012 and February 2013 (based on a few places I just checked in Haiti and Nicaragua whose mapping activity I know for that time period, their download.cloudmade.com page says that the download data is from Dec 2011 but the maps are clearly from after May 2012; perhaps they haven't updated to the OdbL data?). http://maps.cloudmade.com/ is your online URL, if you sign up for an account you can get your personal TMS URL for viewing in various applications (for free for low-volume usage).

Also, Stamen's (beautiful) tiles seem to be from a bit before the license change (spring/summer 2012). E.g. http://maps.stamen.com/#toner/3/23/-45s

There is no easy way to do this, since the infrastructure isn't made to support this.

The Historical OSM community strives to add tags start_date=* to make it possible to see the map at a selected date, but the tag is still very rare, and mostly covers only historical features.

You would need to download a planet file from the timeframe and region that you want. They are updated periodically, but apparently "An experimental full history file is available, containing every revision of each object."

This is the planet file from 14-02-2013, but it's 30 gigabytes: planet-130214.osm.bz2

"MaZderMind is generating extracts of the Full-History-Dump which can be downloaded from his server. If you can't find your own region, read and follow this Tutorial on how to generate your own."

The tutorial is lengthy, and covers only Ubuntu/Linux operating systems, since you would need to run some custom scripts. You can run Python scripts on a Windows/Mac by installing Git first.

All you would need to do is know the coordinates to your area boundaries, and just copy-paste every command from the tutorial into the terminal/Git window.