RSS Playground uses a sample RSS document as a starting point, letting you change the values and create a new document that will remain online for 72 hours.
I used the tool this afternoon to see what the Feed Validator does when it encounters a feed containing RFC 2822 date-time values.
Because this tool's being used to support ongoing development of the proposed specification for RSS, the default values are taken from that proposal rather than the current specification.
If you believe that comments should be accepted without warning, let me know. It would be a simple change.