how to supply a specific timezone to TzSpecificLocalTimeToSystemTime()

Question

The following dBase code invokes a win32 API function to convert a local DST time to a system time. The first parameter set to "null" means that the function takes the current active time zone. What value do I have to put instead of "null" to specify another time zone?

The following page refers to lpTimeZoneInformation as a pointer to a TIME_ZONE_INFORMATION structure that specifies the time zone for the localtime input to this function (lpLocalTime), but is is unclear to me what kind of pointer this is.

I have tried 'Brisbane', 'E. Australia Standard Time', '10:00' and '+10:00' but none returns the expected value.

https://learn.microsoft.com/en-us/windows/win32/api/timezoneapi/nf-timezoneapi-tzspecificlocaltimetosystemtime

ITOH and HTOI are Integer TO Hex and vice-versa conversion functions

localtime and systemtime structures work, I tried to replicate that for the time_zone_information part but without success so far

As it stands, the return value is 13.20

Thanks for any help!

d=new date("31/12/2020 5:08")
offset1=getLocalTimeOffset(d)/60

function getLocalTimeOffset(d_in)
    // todo typechecking of the parameter
    extern clogical TzSpecificLocalTimeToSystemTime(cptr,cptr,cptr) kernel32
    extern culong GetLastError(cvoid) kernel32
    local systemtime,localtime,tmp  
    localtime = replicate(chr(0),16)
    systemtime = replicate(chr(0),16)
    TZI = replicate(chr(0),16)
    TZIa=itoh(-600,4)
    TZIb=itoh(-60,4)
    TZI.setbyte(1,htoi(left(TZIa,2)))
    TZI.setbyte(0,htoi(right(TZIa,2)))  
    TZI.setbyte(9,htoi(left(TZIb,2)))
    TZI.setbyte(8,htoi(right(TZIb,2)))
tmp = itoh(d_in.year,4) 
    localtime.setbyte(1,htoi(left(tmp,2)))      // fill the systemtime structure
    localtime.setbyte(0,htoi(right(tmp,2))) // seconds and ms are of no concern
    localtime.setbyte(2,d_in.month+1)
    localtime.setbyte(4,d_in.day)
    localtime.setbyte(6,d_in.date)
    localtime.setbyte(8,d_in.hour)
    localtime.setbyte(10,d_in.minute)   
    if TzSpecificLocalTimeToSystemTime(TZI,localtime,systemtime) = 0
        tmp = getlasterror() ; ? "Error: "+tmp ; return 9999
    endif
    tmp = sign(d_in.date-systemtime.getbyte(6))*24*60  // consider day boundary
    if (d_in.date = 1 or systemtime.getbyte(6) = 1) and (d_in.month+1 <> systemtime.getbyte(2))
        tmp = -tmp      // adjust for month boundaries
    endif
    tmp += (d_in.hour - systemtime.getbyte(8))*60
    tmp += d_in.minute - systemtime.getbyte(10)
return tmp

Answer 1

(Too long for a comment.)   The first parameter to TzSpecificLocalTimeToSystemTime must be either NULL, or otherwise point to a TIME_ZONE_INFORMATION structure, filled-in with the target timezone data from HKLM\SOFTWARE\Microsoft\Windows NT\CurrentVersion\Time Zones per Remarks on the same page.

In OP's case, Brisbane falls under the E. Australia Standard Time key, and TZI data parses as:

typedef struct _REG_TZI_FORMAT
{
    LONG               Bias;  //  -600    A8 FD FF FF
    LONG       StandardBias;  //     0    00 00 00 00
    LONG       DaylightBias;  //   -60    C4 FF FF FF
    SYSTEMTIME StandardDate;  //   n/a    00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00
    SYSTEMTIME DaylightDate;  //   n/a    00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00
} REG_TZI_FORMAT;

Following is the C code to fill-in a TIME_ZONE_INFORMATION structure with the same data, and successfully convert a Brisbane local time to UTC:

#include <windows.h>
#include <stdio.h>

int main()
{
    TIME_ZONE_INFORMATION tzEAST =          //  [offset]  bytes
    {
         -600,  //  LONG       Bias;                 [0]  A8 FD FF FF
        { 0 },  //  WCHAR      StandardName[32];     [4]  00 .. 00
        { 0 },  //  SYSTEMTIME StandardDate;        [68]  00 .. 00
            0,  //  LONG       StandardBias;        [84]  00 00 00 00
        { 0 },  //  WCHAR      DaylightName[32];    [88]  00 .. 00
        { 0 },  //  SYSTEMTIME DaylightDate;       [152]  00 .. 00
          -60   //  LONG       DaylightBias;       [168]  C4 FF FF FF
    };
    SYSTEMTIME stEAST = { 2021, 1, 1, 4, 12 }, stUTC = { 0 };

    if(!TzSpecificLocalTimeToSystemTime(&tzEAST, &stEAST, &stUTC)) return 1;

    printf("EAST %d-%02d-%02d %02d:%02d:%02d = UTC %d-%02d-%02d %02d:%02d:%02d\n",
           stEAST.wYear, stEAST.wMonth, stEAST.wDay, stEAST.wHour, stEAST.wMinute, stEAST.wSecond,
           stUTC.wYear, stUTC.wMonth, stUTC.wDay, stUTC.wHour, stUTC.wMinute, stUTC.wSecond);

    return 0;
}

Output:

EAST 2021-01-04 12:00:00 = UTC 2021-01-04 02:00:00

---

[ EDIT ]   Following is my guess of what the dBase code might look like. Just a guess, and nothing more than a guess, since I don't actually know dBase beyond what's been posted here.

tzBias    = itoh(-600, 8)
tzDstBias = itoh( -60, 8)

tzi = replicate(chr(0), 86)  // 86*2 = 172 = sizeof TIME_ZONE_INFORMATION

tzi.setbyte(  0, htoi(substring(   tzBias, 6, 8)))  // [  0]  LONG Bias;
tzi.setbyte(  1, htoi(substring(   tzBias, 4, 6)))
tzi.setbyte(  2, htoi(substring(   tzBias, 2, 4)))
tzi.setbyte(  3, htoi(substring(   tzBias, 0, 2)))

tzi.setbyte(168, htoi(substring(tzDstBias, 6, 8)))  // [168]  LONG DaylightBias;
tzi.setbyte(169, htoi(substring(tzDstBias, 4, 6)))
tzi.setbyte(170, htoi(substring(tzDstBias, 2, 4)))
tzi.setbyte(171, htoi(substring(tzDstBias, 0, 2)))

if TzSpecificLocalTimeToSystemTime(tzi, localtime, systemtime) = 0  // ...

[ EDIT #2 courtesy OP ]   The working dBase code to fill the structure is the following:

tzi.setbyte(  0, htoi(substr(tzBias, 7, 2)))  // [  0]  LONG Bias
tzi.setbyte(  1, htoi(substr(tzBias, 5, 2)))
tzi.setbyte(  2, htoi(substr(tzBias, 3, 2)))
tzi.setbyte(  3, htoi(substr(tzBias, 1, 2)))

tzi.setbyte(168, htoi(substr(tzDstBias, 7,2)))  // [168]  LONG DaylightBias
tzi.setbyte(169, htoi(substr(tzDstBias, 5,2)))
tzi.setbyte(170, htoi(substr(tzDstBias, 3,2)))
tzi.setbyte(171, htoi(substr(tzDstBias, 1,2)))