Converting time zone to UTC

[mjolnirmarkiv 1]

Hello there,

I need to convert time from CETCEST to UTC, so naturally I want to take into account that extra hour depending on if summer time is active or not, e.g. 16:00 CET > 15:00 UTC, but from April to October (or whatevere the transition dates are) I need to return 16:00 CEST > 14:00 UTC. My local time is different from CE(S)T, so local to UTC conversion is not an option.

Any suggestions what is the easiest way to do it? Probably sending request to NTP server might help to get the correct offset, but I'm not entirely sure how to do it...

$http = ObjCreate ("winhttp.winhttprequest.5.1")
$http.Open("GET", "http://pool.ntp.org/", false)
$http.Send()
MsgBox (0, "", $http.GetResponseHeader("Date"))

Thanks!

[jchd 984]

If you're going to have to process random timestamps from random places, then the problem is not as simple as you'd like it to be. DST conventions and even timezone boundaries have changed significantly over time and are going to change in the future as well.

That's why it's terribly difficult to convert past dates reliably.

[water 1,806]

I suggest to have a look at the Date UDF that comes with AutoIt.

Functions _Date_Time_SystemTimeToTzSpecificLocalTime and _Date_Time_TzSpecificLocalTimeToSystemTime should do what you want.

[orbs 158]

as jhcd states, time conversion which involves DST may get quite mischievous. as to water's reference, one may find it is a bit more difficult than usual, due to the calling script being required to directly use some DLL structs and pointers. to make this a bit simpler, i made> this UDF. it does not directly support non-local timezones, but that (i believe) should not be too hard to work out.

[jchd 984]

The actual issue with DST is that it has changed (and keeps on changing here and there) a lot over time in various areas. Thanks to political decisions made by people not realizing the consequences, you'd have to maintain a huge table of changes for locations/areas and dates all over the world to perform an accurate conversion. Maybe such table is available somewhere in the netspace but I've no idea where.

Some timezones also have seen their boundaries change in several occurences but I seem to recall that inhabited locations involved where only few and mostly small islands. I may be wrong on this.

In all cases a general purpose correct conversion is anything but simple.

[orbs 158]

Maybe such table is available somewhere in the netspace but I've no idea where.

 

surprisingly enough, it's in the Windows registry. you can browse it in a decently-presented manner using the TZedit utility. it is constantly updated by MS via Windows Update. it may be not properly updated, for any or all of the reasons mentioned above; but for the most part, if your Windows is updated, it can be considered quite a reliable source of information. after all, it's what Windows itself is using. when you change your time zone, Windows does not load the new timezone data - i.e. offset, DST transitions dates, etc - from the web. Windows relies on what is stored in its own registry for that.

[jchd 984]

You misunderstand the problem: the question is not what time shift if any exist now and where you live, but determine what was (or will be) the time shift at a particular date and a particular location on Earth.

The (huge) IANA database is not garanteed to be complete nor correct and only goes back to 1970 and only for major towns. It certainly isn't going to fit in a Windows registry anytime soon...

Also latest time I had to check, Windows didn't maintain such data per user, only machine-wide. This was (is?) problematic for accounts used remotely.

[mjolnirmarkiv 1]

Thanks everyone for the input.

With a little research on how NST servers function, I see now that they return only UTC time, no offsets, no anything but UTC. In other words there's no way to say for sure what is offset for specific time zone.

It seems there's no better solution in converting CE(S)T to UTC but to change bias depending on the date being converted by myself (-60 now or -120 during summer in my case) and hope some bureaucrat or whoever responsible for that won't change DST offset or time of transition.

Smth like this, even though _Date_Time_* functions are not necessity here, simple check for is date between last Sunday of March and last Sunday of October (for CEST) > increase bias to -120 would do the trick.

#include <Array.au3>
#include <Date.au3>

Global $date, $time
_DateTimeSplit ("2014/11/15 15:00", $date, $time)

Global $cet_tzinfo = DllStructCreate ($tagTIME_ZONE_INFORMATION)
DllStructSetData ($cet_tzinfo, "Bias", -60)
;DllStructSetData ($cet_tzinfo, "StdName", "Western Europe (winter)")
;DllStructSetData ($cet_tzinfo, "StdDate", _Date_Time_EncodeSystemTime (10, 5, 0, 3, 0, 0))
;DllStructSetData ($cet_tzinfo, "StdBias", 0)
;DllStructSetData ($cet_tzinfo, "DayName", "Western Europe (summer)")
;DllStructSetData ($cet_tzinfo, "DayDate", _Date_Time_EncodeSystemTime (3, 5, 0, 2, 0, 0))
;DllStructSetData ($cet_tzinfo, "DayBias", -60)

Global $cet = _Date_Time_EncodeSystemTime ($date[2], $date[3], $date[1], $time[1], $time[2], $time[3])
Global $utc = _Date_Time_TzSpecificLocalTimeToSystemTime (DllStructGetPtr ($cet), DllStructGetPtr ($cet_tzinfo))

MsgBox (0, "", _Date_Time_SystemTimeToDateTimeStr ($utc, 1))

Edited November 15, 2014 by mjolnirmarkiv

[mjolnirmarkiv 1]

surprisingly enough, it's in the Windows registry. you can browse it in a decently-presented manner using the TZedit utility. it is constantly updated by MS via Windows Update. it may be not properly updated, for any or all of the reasons mentioned above; but for the most part, if your Windows is updated, it can be considered quite a reliable source of information. after all, it's what Windows itself is using. when you change your time zone, Windows does not load the new timezone data - i.e. offset, DST transitions dates, etc - from the web. Windows relies on what is stored in its own registry for that.

Speaking about registry...

That's not a 100% guarantee, but smth like this I believe? This one correctly converts to UTC depending on the date (within DST -2 hours, otherwise -1 hour).

#include <Array.au3>
#include <Date.au3>

$s = DateTimeToUTC ("2014/10/26 12:00", "Central European Standard Time")
MsgBox (0, @error, $s)

Func DateTimeToUTC ($p_date_time, $p_time_zone)
  Local $bin = RegRead ("HKEY_LOCAL_MACHINE\SOFTWARE\Microsoft\Windows NT\CurrentVersion\Time Zones\" & $p_time_zone, "TZI")
  If @error Then Return SetError (1, 0, $p_date_time)
  If not IsBinary ($bin) or BinaryLen ($bin) <> 44 Then Return SetError (2, 0, $p_date_time)

  Local $tz_info = DllStructCreate ($tagTIME_ZONE_INFORMATION)
  DllStructSetData ($tz_info, "Bias", BinaryMid ($bin, 1, 4))
  DllStructSetData ($tz_info, "StdBias", BinaryMid ($bin, 5, 4))
  DllStructSetData ($tz_info, "DayBias", BinaryMid ($bin, 9, 4))
  $n = 0
  For $i = 13 To 27 Step 2
    $n += 1
    DllStructSetData ($tz_info, "StdDate", BinaryMid ($bin, $i, 2), $n)
  Next
  $n = 0
  For $i = 29 To 43 Step 2
    $n += 1
    DllStructSetData ($tz_info, "DayDate", BinaryMid ($bin, $i, 2), $n)
  Next

  Local $date, $time
  _DateTimeSplit ($p_date_time, $date, $time)
  Local $date_time = _Date_Time_EncodeSystemTime ($date[2], $date[3], $date[1], $time[1], $time[2], $time[3])
  Local $utc = _Date_Time_TzSpecificLocalTimeToSystemTime (DllStructGetPtr ($date_time), DllStructGetPtr ($tz_info))

  Return _Date_Time_SystemTimeToDateTimeStr ($utc, 1)
EndFunc

That was PITA to wrap my head around dll structures and time conversion I must say, more complicated than I thought.

Edited November 15, 2014 by mjolnirmarkiv