What format is your string? Is it always in the same format?
Since you didn't specify what the string looks like, it's a little difficult to supply you with an answer.
That being said, essentially what you'll have to do is parse out the information and pass it to one of the DateTime functions, which come in 3 flavors:
DateTime (date, time)
DateTime (YYYY, MM, DD)
DateTime (YYYY, MM, DD, HH, MM, SS)