RESTful timestamp format: Which would you use?

What timestamp format do you think is best to output from a RESTful API?

milliseconds since 1970 is extremely simple and can be easily converted into any format. Plus, you can compare to current time or another value without even converting at all.

human-readable formats like ISO 8601 (2011-07-14T19:43:37+0100) are much more popular for some reason. It's nice they're human-readable to be sure, but I feel like they always cause a lot of unnecessary parsing hassle.
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