DateTime.ParseExact(input,"yyyyMMdd HH:mm",null);
assuming you meant to say that minutes followed the hours, not seconds – your example is a little confusing.
The ParseExact documentation details other overloads, in case you want to have the parse automatically convert to Universal Time or something like that.
As @Joel Coehoorn mentions, there’s also the option of using TryParseExact, which will return a Boolean value indicating success or failure of the operation – I’m still on .Net 1.1, so I often forget this one.
If you need to parse other formats, you can check out the Standard DateTime Format Strings.