The System.DateTime.Now property returns a System.DateTime. This is stored in memory in a binary format that most programmers in most circumstances have no need to think about. When you display a DateTime value, or convert it to a string for any other reason, it is converted according to a format string, which can specify any format you like.
In this last sense, the answer to your question "if I get DateTime.Now, which Date Time Format is this using?" is "it is not using any DateTime format at all, because you haven't formatted it yet".
You specify the format by calling an overload of ToString, or (optionally) if you use System.String.Format. There is a default format, as well, so you don't always have to specify the format.
string sysFormat = CultureInfo.CurrentCulture.DateTimeFormat;
and apply the same format on your output data time format, pleaselet me know if you want some thing specific.