Test SystemTimeToTzSpecificLocalTime() behavior.

a.cpp

// Test SystemTimeToTzSpecificLocalTime() behavior.
// Does SystemTimeToTzSpecificLocalTime() calculate Daylight Saving Time (DST) with dynamic time zone information?
//
// Test with "Eastern Standard Time"
// 2006: DST start at first Sunday in April
// 2007: DST start at second Sunday in March
//
// tzutil.exe /s "Eastern Standard Time"
// .\a.exe
//
// test1: call with NULL (calculate dst with current dynamic time zone)
//   utc: 2006-03-30 00:00
// local: 2006-03-29 19:00 < STD (UTC-5)
//   utc: 2007-03-30 00:00
// local: 2007-03-29 20:00 < DST (UTC-4)
//
// test2: call with current TIME_ZONE_INFORMATION (calculate dst with current dynamic time zone)
//   utc: 2006-03-30 00:00
// local: 2006-03-29 19:00 < STD (UTC-5)
//   utc: 2007-03-30 00:00
// local: 2007-03-29 20:00 < DST (UTC-4)
//
// test3: call with modified TIME_ZONE_INFORMATION (specified time_zone_information is used)
//   utc: 2006-03-30 00:00
// local: 2006-03-29 19:59 < DST (UTC-4-1m)
//   utc: 2007-03-30 00:00
// local: 2007-03-29 19:59 < DST (UTC-4-1m)

#include <windows.h>
#include <print>

void test1(WORD year, WORD month, WORD day, WORD hour, WORD minute)
{
    SYSTEMTIME st_utc { year, month, 0, day, hour, minute, 0, 0 };
    SYSTEMTIME st_local;

    if (!SystemTimeToTzSpecificLocalTime(nullptr, &st_utc, &st_local))
    {
        std::println("test1 fails");
        return;
    }

    std::println("  utc: {:04}-{:02}-{:02} {:02}:{:02}", st_utc.wYear, st_utc.wMonth, st_utc.wDay, st_utc.wHour, st_utc.wMinute);
    std::println("local: {:04}-{:02}-{:02} {:02}:{:02}", st_local.wYear, st_local.wMonth, st_local.wDay, st_local.wHour, st_local.wMinute);
}

void test2(WORD year, WORD month, WORD day, WORD hour, WORD minute)
{
    TIME_ZONE_INFORMATION tzi;
    SYSTEMTIME st_utc { year, month, 0, day, hour, minute, 0, 0 };
    SYSTEMTIME st_local;

    if (GetTimeZoneInformation(&tzi) == TIME_ZONE_ID_INVALID)
    {
        std::println("test2 fails");
        return;
    }

    if (!SystemTimeToTzSpecificLocalTime(&tzi, &st_utc, &st_local))
    {
        std::println("test2 fails");
        return;
    }

    std::println("  utc: {:04}-{:02}-{:02} {:02}:{:02}", st_utc.wYear, st_utc.wMonth, st_utc.wDay, st_utc.wHour, st_utc.wMinute);
    std::println("local: {:04}-{:02}-{:02} {:02}:{:02}", st_local.wYear, st_local.wMonth, st_local.wDay, st_local.wHour, st_local.wMinute);
}

void test3(WORD year, WORD month, WORD day, WORD hour, WORD minute)
{
    TIME_ZONE_INFORMATION tzi;
    SYSTEMTIME st_utc { year, month, 0, day, hour, minute, 0, 0 };
    SYSTEMTIME st_local;

    if (GetTimeZoneInformation(&tzi) == TIME_ZONE_ID_INVALID)
    {
        std::println("test3 fails");
        return;
    }

    tzi.Bias += 1;

    if (!SystemTimeToTzSpecificLocalTime(&tzi, &st_utc, &st_local))
    {
        std::println("test3 fails");
        return;
    }

    std::println("  utc: {:04}-{:02}-{:02} {:02}:{:02}", st_utc.wYear, st_utc.wMonth, st_utc.wDay, st_utc.wHour, st_utc.wMinute);
    std::println("local: {:04}-{:02}-{:02} {:02}:{:02}", st_local.wYear, st_local.wMonth, st_local.wDay, st_local.wHour, st_local.wMinute);
}

int main()
{
    test1(2006, 3, 30, 0, 0);
    test1(2007, 3, 30, 0, 0);

    std::println("");

    test2(2006, 3, 30, 0, 0);
    test2(2007, 3, 30, 0, 0);

    std::println("");

    test3(2006, 3, 30, 0, 0);
    test3(2007, 3, 30, 0, 0);

    return 0;
}