Palmsey, I have just recently had the same requirment so I went ahead and wrote a simple parser. Its not the nicest code but it will handle things like:
"Today at 2pm"
"Tuesday at 2pm - 15th july 2010 at 2am"
"Previous Year at 2am - Tommorow at 14:30"
"18th july 2010 at 2:45pm"
Stuck it on codeplex as maybe someone else will find it useful. Check it out: http://timestamper.codeplex.com/
