I am looking for a website that allows me to retrieve the time at which each difficulty change happened. The time doesn't need to be accurate so a time stamp from the block will do. I want to fetch this information automatically every now and then, but I can't seem to find a webpage that provides an API for this.
http://blockexplorer.com/q/nethash/2016 provides the necessary information and shouldn't be too hard to parse. Every row corresponds to the first block at a given difficulty.