I've checked several frameworks and Python's ParseDateTime worked the best. It can be used from .NET using IronPython.
If anyone's interested in a full sample project, comment on the answer and I'll try to create one.
EDIT
As requested, here is a simple project that you can use with the library:
http://www.assembla.com/code/relativedateparser/subversion/nodes
Try the following usage case, for example:
- August 25th, 2008
- 25 Aug 2008
- Aug 25 5pm
- 5pm August 25
- next saturday
- tomorrow
- next thursday at 4pm
- at 4pm
- eod
- tomorrow eod
- eod tuesday
- eoy
- eom
- in 5 minutes
- 5 minutes from now
- 5 hours before now
- 2 hours before noon
- 2 days from tomorrow