If you have a number of feeds from multiple sources (blogs, news sites, etc.) and want to merge them to one feed I have ready solution for you. It can be done very easy and quickly with Yahoo Pipes. Just create “Fetch Feed” module in new just created pipe and add your feeds to it like on the image below.

If you have a lot of feeds you may separate them into a few “Fetch Feed” modules and then use “Union” module like on the image below.

Finally you can sort the items of result feed by publish date using “Sort” module like on the image below.

As a result we got the feed containing the items of multiple feeds and the items are sorted by publish date. You can find this pipe here.