FWIW, Ruby's chronic contains some date formats that this doesn't seem to support right now ('5 minutes ago'):
Aside: I'd really like to see a publicly-available set of natural language date test cases shared across these projects.
The chronic project was actually the original motivation behind natty. I think the chronic project is great, but I believe the grammar-based, AST approach is the way to go for long-term maintainability.
Implementing relative times has been on my list of things to do (in addition to recurrence.) I created a feature request here: http://github.com/joestelmach/natty/issues#issue/4, so feel free to list any time formats you'd like to see implemented.
I agree that a generic list of test cases would be nice. Any thoughts on how such a list should be published?
I guess such a list of test-cases could simply be a JSON file containing the input strings plus an output specification. The options for output would only be explicit datetimes, datetimes relative to the now, or timespans.