You can't convert a Timespan to a DateTime: that doesn't work because it's not a logical action. Think about it: If I ask you to meet me at "plus 6 hours 30 minutes" what does that mean? Nothing - unless it's understood that there is a starting point for it to be relative to. In terms of normal conversation, "I'll meet you in thirty minutes" is fine - it's understood by all parties that that means "thirty minutes from now" but without that understanding we will all miss the appointment.
In computing terms you can include the current time very easily:
TimeSpan a = new TimeSpan(12, 00, 00);
DateTime dt = DateTime.Now + a;
But you can't expect a TimeSpan to convert directly.