date format yyyy-MM-ddTHH:mm:ssZ

Using UTC

ISO 8601 (MSDN datetime formats)

Console.WriteLine(DateTime.UtcNow.ToString("s") + "Z");

2009-11-13T10:39:35Z

The Z is there because

If the time is in UTC, add a 'Z' directly after the time without a space. 'Z' is the zone designator for the zero UTC offset. "09:30 UTC" is therefore represented as "09:30Z" or "0930Z". "14:45:15 UTC" would be "14:45:15Z" or "144515Z".

If you want to include an offset

int hours = TimeZoneInfo.Local.BaseUtcOffset.Hours;
string offset = string.Format("{0}{1}",((hours >0)? "+" :""),hours.ToString("00"));
string isoformat = DateTime.Now.ToString("s") + offset;
Console.WriteLine(isoformat);

Two things to note: + or - is needed after the time but obviously + doesn't show on positive numbers. According to wikipedia the offset can be in +hh format or +hh:mm. I've kept to just hours.

As far as I know, RFC1123 (HTTP date, the "u" formatter) isn't meant to give time zone offsets. All times are intended to be GMT/UTC.

Console.WriteLine(DateTime.UtcNow.ToString("o"));  
Console.WriteLine(DateTime.Now.ToString("o"));

Outputs:

2012-07-09T19:22:09.1440844Z  
2012-07-09T12:22:09.1440844-07:00

"o" format is different for DateTime vs DateTimeOffset :(

DateTime.UtcNow.ToString("o") -> "2016-03-09T03:30:25.1263499Z"

DateTimeOffset.UtcNow.ToString("o") -> "2016-03-09T03:30:46.7775027+00:00"

My final answer is

DateTimeOffset.UtcDateTime.ToString("o")   //for DateTimeOffset type
DateTime.UtcNow.ToString("o")              //for DateTime type