c++ - How do you get info for an arbitrary time zone in Windows?

Question

Ideally, what I'd like to be able to do is take the name of a time zone and ask Windows for its corresponding time zone info (offset from UTC, DST offset, dates for DST switch, etc.). It looks like Windows uses a TIME_ZONE_INFORMATION struct to hold this sort of info. So, presumably, I want a function which takes a string with the time zone's name and returns a TIME_ZONE_INFORMATION struct.

However, all I can find are functions such as GetTimeZoneInformation() which give me the TIME_ZONE_INFORMATION for the local time. What I need is a function which will give me that information for an arbitrary time zone regardless of what the local time zone is.

The only way that I see to get that information is to go grab it directly from the registry, which is less than ideal. The TIME_ZONE_INFORMATION page shows where it is in the registry, so it should be possible to fetch the information from there, but I'd much prefer a proper system function for doing it. Does such a function exist, or do I have to go registry diving to get the time zone info for an arbitrary time zone?

Answer 1

The time zone information is contained as binary data in the registry under HKEY_LOCAL_MACHINESOFTWAREMicrosoftWindows NTCurrentVersionTime Zones(zone name)TZI. The structure of the data is given in the TIME_ZONE_INFORMATION documentation:

struct STimeZoneFromRegistry
{
 long  Bias;
 long  StandardBias;
 long  DaylightBias;
 SYSTEMTIME StandardDate;
 SYSTEMTIME DaylightDate;
};

And here's example code to read the key:

TIME_ZONE_INFORMATION tz = {0};
STimeZoneFromRegistry binary_data;
DWORD size = sizeof(binary_data);
HKEY hk = NULL;
TCHAR zone_key[] = _T("SOFTWARE\Microsoft\Windows NT\CurrentVersion\Time Zones\Central Standard Time");
if ((RegOpenKeyEx(HKEY_LOCAL_MACHINE, zone_key, 0, KEY_QUERY_VALUE, &hk) == ERROR_SUCCESS) &&
 (RegQueryValueEx(hk, "TZI", NULL, NULL, (BYTE *) &binary_data, &size) == ERROR_SUCCESS))
{
 tz.Bias = binary_data.Bias;
 tz.DaylightBias = binary_data.DaylightBias;
 tz.DaylightDate = binary_data.DaylightDate;
 tz.StandardBias = binary_data.StandardBias;
 tz.StandardDate = binary_data.StandardDate;
}

Edit: Sorry, this answer is redundant - I'm sure you could have figured all this out using the documentation you linked to in the question. I've only had to do this once, and this is the only method I could find.