How to parse relative time?

This question is the other side of the question asking, "How do I calculate relative time?".

Given some human input for a relative time, how can you parse it? By default you would offset from DateTime.Now(), but could optionally offset from another DateTime.

(Prefer answers in C#)

Example input:

• "in 20 minutes"
• "5 hours ago"
• "3h 2m"
• "next week"

Edit: Let's suppose we can define some limits on the input. This sort of code would be a useful thing to have out on the web.