A Google search turns up the parsedatetime library (associated with the Chandler project), which is designed to do exactly this. It's open source (Apache License) and written in Python. It seems to be quite sophisticated -- from the homepage:
parsedatetime is able to parse, for
example, the following:
* Aug 25 5pm
* 5pm August 25
* next saturday
...
* tomorrow
* next thursday at 4pm
* at 4pm
* eod
* in 5 minutes
* 5 minutes from now
* 5 hours before now
* 2 days from tomorrow
Since it's implemented in pure Python and doesn't use anything fancy, there's a good chance it's compatible with IronPython, so you could use it with .net. If you want specifically a C# solution, you could write something based on the algorithms they use...
It also comes with a whole bunch of unit tests.
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