Как конвертировать RFC дату и обратно?

function DateTimeToRfcTime(
  dt: TDateTime;
  iDiff: integer;
  blnGMT: boolean = false): string;
{*
Explanation:
iDiff is the local offset to GMT in minutes
if blnGMT then Result is UNC time else local time
e.g. local time zone: ET = GMT - 5hr = -300 minutes
    dt is TDateTime of 3 Jan 2001 5:45am
      blnGMT = true  -> Result = 'Wed, 03 Jan 2001 05:45:00 GMT'
      blnGMT = false -> Result = 'Wed, 03 Jan 2001 05:45:00 -0500'
*}
const
  Weekday: array[1..7] of string =
  ('Sun', 'Mon', 'Tue', 'Wed', 'Thu', 'Fri', 'Sat');
  Month: array[1..12] of string = (
    'Jan', 'Feb', 'Mar', 'Apr', 'May', 'Jun',
    'Jul', 'Aug', 'Sep', 'Oct', 'Nov', 'Dec');
var
  iDummy: Word;
  iYear: Word;
  iMonth: Word;
  iDay: Word;
  iHour: Word;
  iMinute: Word;
  iSecond: Word;
  strZone: string;
begin
  if blnGMT then
  begin
    dt := dt - iDiff / 1440;
    strZone := 'GMT';
  end
  else
  begin
    iDiff := (iDiff div 60) * 100 + (iDiff mod 60);
    if iDiff < 0 then
      strZone := Format('-%.4d', [-iDiff])
    else
      strZone := Format('+%.4d', [iDiff]);
  end;
  DecodeDate(dt, iYear, iMonth, iDay);
  DecodeTime(dt, iHour, iMinute, iSecond, iDummy);
  Result := Format('%s, %.2d %s %4d %.2d:%.2d:%.2d %s', [
    Weekday[DayOfWeek(dt)], iDay, Month[iMonth], iYear,
      iHour, iMinute, iSecond, strZone]);
end;
 
function RfcTimeToDateTime(
  strTime: string;
  blnGMT: boolean = true): TDateTime;
{*
Explanation:
if blnGMT then Result is UNC time else local time
e.g. local time zone: ET = GMT - 5hr = -0500
    strTime = 'Wed, 03 Jan 2001 05:45:00 -0500'
      blnGMT = true  -> FormatDateTime('...', Result) = '03.01.2001 10:45:00'
      blnGMT = false -> FormatDateTime('...', Result) = '03.01.2001 05:45:00'
*}
const
  wd = 'sun#mon#tue#wed#thu#fri#sat';
  month = 'janfebmaraprmayjunjulaugsepoctnovdec';
var
  s: string;
  dd: Word;
  mm: Word;
  yy: Word;
  hh: Word;
  nn: Word;
  ss: Word;
begin
  s := LowerCase(Copy(strTime, 1, 3));
  if Pos(s, wd) > 0 then
    Delete(strTime, 1, Pos(' ', strTime));
  s := Trim(Copy(strTime, 1, Pos(' ', strTime)));
  Delete(strTime, 1, Length(s) + 1);
  dd := StrToIntDef(s, 0);
  s := LowerCase(Copy(strTime, 1, 3));
  Delete(strTime, 1, 4);
  mm := (Pos(s, month) div 3) + 1;
  s := Copy(strTime, 1, 4);
  Delete(strTime, 1, 5);
  yy := StrToIntDef(s, 0);
  Result := EncodeDate(yy, mm, dd);
  s := strTime[1] + strTime[2];
  hh := StrToIntDef(strTime[1] + strTime[2], 0);
  nn := StrToIntDef(strTime[4] + strTime[5], 0);
  ss := StrToIntDef(strTime[7] + strTime[8], 0);
  Delete(strTime, 1, 9);
  Result := Result + EncodeTime(hh, nn, ss, 0);
  if (CompareText(strTime, 'gmt') <> 0) and blnGMT then
  begin
    hh := StrToIntDef(strTime[2] + strTime[3], 0);
    nn := StrToIntDef(strTime[4] + strTime[5], 0);
    if strTime[1] = '+' then
      Result := Result - EncodeTime(hh, nn, 0, 0)
    else
      Result := Result + EncodeTime(hh, nn, 0, 0);
  end;
end;

Взято с Delphi Knowledge Base: https://www.baltsoft.com/

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; 

Взято с сайта https://www.swissdelphicenter.ch/en/tipsindex.php