Convert TDateTime to Another Time Zone Regardless of Local Time Zone

Question

Regardless of what the user's local time zone is set to, using Delphi 2007, I need to determine the time (TDateTime) in the Eastern time zone.

How can I do that? Of course, needs to be daylight savings time aware.

Answer 1

TDateTime does not have any time zone information (it is just a double - date as whole number, time as decimal), so you would need that separately. You would need your own logic for DST as well, I don't believe there is any in Delphi. Then use the IncHour function in DateUtils.pas to alter the TDateTime to the Eastern Timezone.

There are probably web services that will do this for you. Does your application need to be self contained, or can it connect to the web to do it?

Answer 2

To be specific, TDateTime isn't object, it's just an alias for double.

Answer 3

If I understand you correctly, you want the Eastern Time equivalent of the current system time.

To do this, use the WiNAPI function GetSystemTime() to get the current time of the computer in UTC. UTC is independent of time zones and will always get you the time at the prime meridian.

You can then use the WinAPI function SystemTimeToTzSpecificLocalTime() to calculate the local time in any other given time zone from the UTC time. In order that SystemTimeToTzSpecificLocalTime() can work, you need to give it a TTimeZoneInformation record that is populated with the correct information for the time zone you want to convert to.

The following sample will always gives you the local time in Eastern Time as per Energy Policy Act of 2005.

function GetEasternTime: TDateTime;
var
  T: TSystemTime;
  TZ: TTimeZoneInformation;
begin
  // Get Current time in UTC
  GetSystemTime(T);

  // Setup Timezone Information for Eastern Time
  TZ.Bias:= 0;

  // DST ends at First Sunday in November at 2am
  TZ.StandardBias:= 300;
  TZ.StandardDate.wYear:= 0;
  TZ.StandardDate.wMonth:= 11; // November
  TZ.StandardDate.wDay:= 1; // First
  TZ.StandardDate.wDayOfWeek:= 0; // Sunday
  TZ.StandardDate.wHour:= 2;
  TZ.StandardDate.wMinute:= 0;
  TZ.StandardDate.wSecond:= 0;
  TZ.StandardDate.wMilliseconds:= 0;

  // DST starts at Second Sunday in March at 2am
  TZ.DaylightBias:= 240;
  TZ.DaylightDate.wYear:= 0;
  TZ.DaylightDate.wMonth:= 3; // March
  TZ.DaylightDate.wDay:= 2; // Second
  TZ.DaylightDate.wDayOfWeek:= 0; // Sunday
  TZ.DaylightDate.wHour:= 2;
  TZ.DaylightDate.wMinute:= 0;
  TZ.DaylightDate.wSecond:= 0;
  TZ.DaylightDate.wMilliseconds:= 0;

  // Convert UTC to Eastern Time
  Win32Check(SystemTimeToTzSpecificLocalTime(@TZ, T, T));

  // Convert to and return as TDateTime
  Result := EncodeDate(T.wYear, T.wMonth, T.wDay) + 
   EncodeTime(T.wHour, T.wMinute, T.wSecond, T.wMilliSeconds);
end;

procedure TForm1.Button1Click(Sender: TObject);
begin
  Label1.Caption:= 'In New York Citiy, it is now ' + DateTimeToStr(GetEasternTime);
end;