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 sides should be able to exchange Unix timestamps (in UTC, without DST) and be able to communicate through this way.

In a bugreport of HeidiSQL , users discussed that DateTimeToUnix and UnixToDateTime do not care about the time zone, and there I have found following code:

function DateTimeToUTC(dt: TDateTime): Int64;
var
  tzi: TTimeZoneInformation;
begin
  Result := DateTimeToUnix(dt);
  GetTimeZoneInformation(tzi);
  Result := Result + tzi.Bias * 60;
end;

MSDN explains twi.Bias as follows:

All translations between UTC time and local time are based on the following formula:

UTC = local time + bias

The bias is the difference, in minutes, between UTC time and local time.

This sounds logical, but since I was unsure if the code above was correct, I made following program to check it:

// A date in summer time (DST)
Memo1.Lines.add('1401494400'); // 31 May 2014 00:00:00 GMT according to http://www.epochconverter.com/
Memo1.Lines.add(inttostr(DateTimeToUnixUTC(StrToDate('31.05.2014'))));

// A date in winter time
Memo1.Lines.add('567302400'); // 24 Dec 1987 00:00:00 GMT according to http://www.epochconverter.com/
Memo1.Lines.add(inttostr(DateTimeToUnixUTC(StrToDate('24.12.1987'))));

The output in Germany (GMT+1+DST) is currently:

1401494400
1401490800
567302400
567298800

I expected the output being:

1401494400
1401494400
567302400
567302400

What am I doing wrong?

PS: For this project I am bound to Delphi 6.

Answer 1

You have already found DateTimeToUnix and UnixToDateTime. So that part of the conversion is taken care of.

All you need to do now is convert between local and UTC time. You can do that using DateUtils.TTimeZone class. Specifically DateUtils.TTimeZone.ToUniversalTime and DateUtils.TTimeZone.ToLocalTime.

These four functions give you all that you need.