Seltzer

public static class DateTimes
{
    const string W3CDateTimeOffsetFormatString = "yyyy-MM-ddTHH:mm:sszzz";
    const string W3CDateTimeFormatString = "yyyy-MM-ddTHH:mm:ss";

    public static string ToW3COffset(DateTimeOffset? dto)
    {
        return dto.HasValue ? dto.Value.ToString(W3CDateTimeOffsetFormatString) : null;
    }