Pretty easy really - DateTime has a constructor that takes a Ticks count:
http://msdn.microsoft.com/en-us/library/z2xf7zzk(v=vs.110).aspx[
^] so all you need to do is generate a random Int64:
public static Int64 NextInt64(this Random rand)
{
var buffer = new byte[sizeof(Int64)];
rand.NextBytes(buffer);
return BitConverter.ToInt64(buffer, 0);
}
You might have to fiddle a bit to get "suitable" values - but that's down to your date range anyway!