Автор: http://www.swissdelphicenter.ch

function RFC1123ToDateTime(Date: string): TDateTime;
var
  day, month, year: Integer;
  strMonth: string;
  Hour, Minute, Second: Integer;
begin
  try
    day := StrToInt(Copy(Date, 6, 2));
    strMonth := Copy(Date, 9, 3);
    if strMonth = 'Jan' then
      month := 1
    else if strMonth = 'Feb' then
      month := 2
    else if strMonth = 'Mar' then
      month := 3
    else if strMonth = 'Apr' then
      month := 4
    else if strMonth = 'May' then
      month := 5
    else if strMonth = 'Jun' then
      month := 6
    else if strMonth = 'Jul' then
      month := 7
    else if strMonth = 'Aug' then
      month := 8
    else if strMonth = 'Sep' then
      month := 9
    else if strMonth = 'Oct' then
      month := 10
    else if strMonth = 'Nov' then
      month := 11
    else if strMonth = 'Dec' then
      month := 12;
    year := StrToInt(Copy(Date, 13, 4));
    hour := StrToInt(Copy(Date, 18, 2));
    minute := StrToInt(Copy(Date, 21, 2));
    second := StrToInt(Copy(Date, 24, 2));
    Result := 0;
    Result := EncodeTime(hour, minute, second, 0);
    Result := Result + EncodeDate(year, month, day);
  except
    Result := now;
  end;
end;

function DateTimeToRFC1123(aDate: TDateTime): string;
const
  StrWeekDay: string = 'MonTueWedThuFriSatSun';
  StrMonth: string = 'JanFebMarAprMayJunJulAugSepOctNovDec';
var
  Year, Month, Day: Word;
  Hour, Min, Sec, MSec: Word;
  DayOfWeek: Word;
begin
  DecodeDate(aDate, Year, Month, Day);
  DecodeTime(aDate, Hour, Min, Sec, MSec);
  DayOfWeek := ((Trunc(aDate) - 2) mod 7);
  Result := Copy(StrWeekDay, 1 + DayOfWeek * 3, 3) + ', ' +
    Format('%2.2d %s %4.4d %2.2d:%2.2d:%2.2d',
    [Day, Copy(StrMonth, 1 + 3 * (Month - 1), 3),
    Year, Hour, Min, Sec]);
end;