Добавить значения из второго столбца в зависимости от значений с помощью раскрывающегося списка в первом

Necromancing. Здесь много плохих ответов.

Вот как:

1. Получить строку (предварительное условие: формат: ddd, dd MMM yyyy HH: mm: ss zzz)
2. Получить last whitespace
3. Удалить zzz из строки, но сохранить значение zzz
4. Смещение поиска для zzz
5. Добавить смещение в строку

string dateString = reader.ReadContentAsString();
int timeZonePos = dateString.LastIndexOf(' ') + 1;
string tz = dateString.Substring(timeZonePos);
dateString = dateString.Substring(0, dateString.Length - tz.Length );
dateString += s_timeZoneOffsets[tz];

// https://msdn.microsoft.com/en-us/library/w2sa9yss(v=vs.110).aspx
//string es = reader.ReadElementString("pubDate");
this.m_value = System.DateTime.ParseExact(dateString, "ddd, dd MMM yyyy HH:mm zzz", System.Globalization.CultureInfo.InvariantCulture);

с

private static System.Collections.Generic.Dictionary<string, string> s_timeZoneOffsets =
    new System.Collections.Generic.Dictionary<string, string>() {
    {"ACDT", "+10:30"},
    {"ACST", "+09:30"},
    {"ADT", "-03:00"},
    {"AEDT", "+11:00"},
    {"AEST", "+10:00"},
    {"AHDT", "-09:00"},
    {"AHST", "-10:00"},
    {"AST", "-04:00"},
    {"AT", "-02:00"},
    {"AWDT", "+09:00"},
    {"AWST", "+08:00"},
    {"BAT", "+03:00"},
    {"BDST", "+02:00"},
    {"BET", "-11:00"},
    {"BST", "-03:00"},
    {"BT", "+03:00"},
    {"BZT2", "-03:00"},
    {"CADT", "+10:30"},
    {"CAST", "+09:30"},
    {"CAT", "-10:00"},
    {"CCT", "+08:00"},
    {"CDT", "-05:00"},
    {"CED", "+02:00"},
    {"CET", "+01:00"},
    {"CEST", "+02:00"},
    {"CST", "-06:00"},
    {"EAST", "+10:00"},
    {"EDT", "-04:00"},
    {"EED", "+03:00"},
    {"EET", "+02:00"},
    {"EEST", "+03:00"},
    {"EST", "-05:00"},
    {"FST", "+02:00"},
    {"FWT", "+01:00"},
    {"GMT", "+00:00"},
    {"GST", "+10:00"},
    {"HDT", "-09:00"},
    {"HST", "-10:00"},
    {"IDLE", "+12:00"},
    {"IDLW", "-12:00"},
    {"IST", "+05:30"},
    {"IT", "+03:30"},
    {"JST", "+09:00"},
    {"JT", "+07:00"},
    {"MDT", "-06:00"},
    {"MED", "+02:00"},
    {"MET", "+01:00"},
    {"MEST", "+02:00"},
    {"MEWT", "+01:00"},
    {"MST", "-07:00"},
    {"MT", "+08:00"},
    {"NDT", "-02:30"},
    {"NFT", "-03:30"},
    {"NT", "-11:00"},
    {"NST", "+06:30"},
    {"NZ", "+11:00"},
    {"NZST", "+12:00"},
    {"NZDT", "+13:00"},
    {"NZT", "+12:00"},
    {"PDT", "-07:00"},
    {"PST", "-08:00"},
    {"ROK", "+09:00"},
    {"SAD", "+10:00"},
    {"SAST", "+09:00"},
    {"SAT", "+09:00"},
    {"SDT", "+10:00"},
    {"SST", "+02:00"},
    {"SWT", "+01:00"},
    {"USZ3", "+04:00"},
    {"USZ4", "+05:00"},
    {"USZ5", "+06:00"},
    {"USZ6", "+07:00"},
    {"UT", "-00:00"},
    {"UTC", "-00:00"},
    {"UZ10", "+11:00"},
    {"WAT", "-01:00"},
    {"WET", "-00:00"},
    {"WST", "+08:00"},
    {"YDT", "-08:00"},
    {"YST", "-09:00"},
    {"ZP4", "+04:00"},
    {"ZP5", "+05:00"},
    {"ZP6", "+06:00"}
};