DateTimeToUnix in UTC?

Question

I need UTC variants of the functions DateTimeToUnix and UnixToDateTime, so a Chinese customer is able to interact with the server in Germany. Both

Answer 1

I think I have found some solutions for my question. All 3 solutions gave the same output, but I will try to find out which one is best and I will test it on several machines with different locales.

Solution #1 using TzSpecificLocalTimeToSystemTime and SystemTimeToTzSpecificLocalTime works fine, but requires Windows XP and above:

(Source: https://stackoverflow.com/a/15567777/3544341 , modified)

// Statically binds Windows API functions instead of calling them dynamically.
// Requires Windows XP for the compiled application to run.
{.$DEFINE USE_NEW_WINDOWS_API}

{$IFDEF USE_NEW_WINDOWS_API}
function SystemTimeToTzSpecificLocalTime(lpTimeZoneInformation: PTimeZoneInformation; var lpUniversalTime,lpLocalTime: TSystemTime): BOOL; stdcall; external kernel32 name 'SystemTimeToTzSpecificLocalTime';
{$ELSE}
function SystemTimeToTzSpecificLocalTime(lpTimeZoneInformation: PTimeZoneInformation; var lpUniversalTime,lpLocalTime: TSystemTime): BOOL; stdcall;
var
  h: HModule;
  f: function(lpTimeZoneInformation: PTimeZoneInformation; var lpUniversalTime,lpLocalTime: TSystemTime): BOOL; stdcall;
begin
  h := LoadLibrary(kernel32);
  if h = 0 then RaiseLastOSError;

  @f := GetProcAddress(h, 'SystemTimeToTzSpecificLocalTime');
  if @f = nil then RaiseLastOSError;

  result := f(lpTimeZoneInformation, lpUniversalTime, lpLocalTime);
end;
{$ENDIF}

{$IFDEF USE_NEW_WINDOWS_API}
function TzSpecificLocalTimeToSystemTime(lpTimeZoneInformation: PTimeZoneInformation; var lpLocalTime, lpUniversalTime: TSystemTime): BOOL; stdcall; external kernel32 name 'TzSpecificLocalTimeToSystemTime';
{$ELSE}
function TzSpecificLocalTimeToSystemTime(lpTimeZoneInformation: PTimeZoneInformation; var lpLocalTime, lpUniversalTime: TSystemTime): BOOL; stdcall;
var
  h: HModule;
  f: function(lpTimeZoneInformation: PTimeZoneInformation; var lpLocalTime, lpUniversalTime: TSystemTime): BOOL; stdcall;
begin
  h := LoadLibrary(kernel32);
  if h = 0 then RaiseLastOSError;

  @f := GetProcAddress(h, 'TzSpecificLocalTimeToSystemTime');
  if @f = nil then RaiseLastOSError;

  result := f(lpTimeZoneInformation, lpLocalTime, lpUniversalTime);
end;
{$ENDIF}

function UTCToLocalDateTime_WinXP(d: TDateTime): TDateTime;
var
  TZI: TTimeZoneInformation;
  LocalTime, UniversalTime: TSystemTime;
begin
  GetTimeZoneInformation(tzi);
  DateTimeToSystemTime(d,UniversalTime);
  SystemTimeToTzSpecificLocalTime(@tzi,UniversalTime,LocalTime);
  Result := SystemTimeToDateTime(LocalTime);
end;

function LocalDateTimeToUTC_WinXP(d: TDateTime): TDateTime;
var
  TZI: TTimeZoneInformation;
  LocalTime, UniversalTime: TSystemTime;
begin
  GetTimeZoneInformation(tzi);
  DateTimeToSystemTime(d,LocalTime);
  TzSpecificLocalTimeToSystemTime(@tzi,LocalTime,UniversalTime);
  Result := SystemTimeToDateTime(UniversalTime);
end;

Solution #2 as workaround for older operating systems does also work fine:

(Source: http://www.delphipraxis.net/299286-post4.html )

uses DateUtils;

function GetDateTimeForBiasSystemTime(GivenDateTime: TSystemTime; GivenYear: integer): TDateTime;
var
  Year, Month, Day: word;
  Hour, Minute, Second, MilliSecond: word;
begin
  GivenDateTime.wYear := GivenYear;
  while not TryEncodeDayOfWeekInMonth(GivenDateTime.wYear, GivenDateTime.wMonth, GivenDateTime.wDay, GivenDateTime.wDayOfWeek, Result) do
    Dec(GivenDateTime.wDay);

  DecodeDateTime(Result, Year, Month, Day, Hour, Minute, Second, MilliSecond);
  Result := EncodeDateTime(Year, Month, Day, GivenDateTime.wHour, GivenDateTime.wMinute, GivenDateTime.wSecond, GivenDateTime.wMilliseconds);
end;

function GetBiasForDate(GivenDateTime: TDateTime): integer;
var
  tzi: TIME_ZONE_INFORMATION;
begin
  GetTimeZoneInformation(tzi);
  if (GivenDateTime < GetDateTimeForBiasSystemTime(tzi.StandardDate, YearOf(GivenDateTime))) and
     (GivenDateTime >= GetDateTimeForBiasSystemTime(tzi.DaylightDate, YearOf(GivenDateTime))) then
    Result := (tzi.Bias + tzi.DaylightBias) * -1
  else
    Result := (tzi.Bias + tzi.StandardBias) * -1;
end;

function UTCToLocalDateTime_OldWin(aUTC: TDateTime): TDateTime;
begin
  Result := IncMinute(aUTC, GetBiasForDate(aUTC));
end;

function LocalDateTimeToUTC_OldWin(aLocal: TDateTime): TDateTime;
begin
  Result := IncMinute(aLocal, GetBiasForDate(aLocal) * -1);
end;

Solution #3 using TTimeZone for users of newer versions of Delphi, does give the same results as the codes above:

(Solution by David Heffernan, alas not possible in my current project, because I am bound to Delphi 6)

uses DateUtils;

{$IF Declared(TTimeZone)}
function UTCToLocalDateTime_XE(aUTC: TDateTime): TDateTime;
begin
  result := TTimeZone.Local.ToLocalTime(aUTC);
end;

function LocalDateTimeToUTC_XE(aLocal: TDateTime): TDateTime;
begin
  result := TTimeZone.Local.ToUniversalTime(aLocal);
end;
{$IFEND}

Now we can put all 3 solutions together! :-)

function UTCToLocalDateTime(aUTC: TDateTime): TDateTime;
begin
  {$IF Declared(UTCToLocalDateTime_XE)}
  result := UTCToLocalDateTime_XE(aUTC);
  {$ELSE}
    {$IFDEF USE_NEW_WINDOWS_API}
    result := UTCToLocalDateTime_WinXP(aUTC);
    {$ELSE}
    try
      result := UTCToLocalDateTime_WinXP(aUTC);
    except
      on E: EOSError do
      begin
        // Workaround for Windows versions older than Windows XP
        result := UTCToLocalDateTime_OldWin(aUTC);
      end
      else raise;
    end;
    {$ENDIF}
  {$IFEND}
end;

function LocalDateTimeToUTC(aLocal: TDateTime): TDateTime;
begin
  {$IF Declared(LocalDateTimeToUTC_XE)}
  result := LocalDateTimeToUTC_XE(aLocal);
  {$ELSE}
    {$IFDEF USE_NEW_WINDOWS_API}
    result := LocalDateTimeToUTC_WinXP(aLocal);
    {$ELSE}
    try
      result := LocalDateTimeToUTC_WinXP(aLocal);
    except
      on E: EOSError do
      begin
        // Workaround for Windows versions older than Windows XP
        result := LocalDateTimeToUTC_OldWin(aLocal);
      end
      else raise;
    end;
    {$ENDIF}
  {$IFEND}
end;

---

An easy method to get the current UTC unix timestamp is

function NowUTC: TDateTime;
var
  st: TSystemTime;
begin
  GetSystemTime(st);
  result := EncodeDateTime(st.wYear, st.wMonth, st.wDay, st.wHour, st.wMinute, st.wSecond, st.wMilliseconds);
end;

function CurrentUnixUTCTimestamp: int64;
begin
  result := DateTimeToUnix(NowUTC);
end;