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Universal Time Converter

, 2 Jan 2006
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Illustrates converting time from universal time to local time and vice versa in C++
Sample Image - TimeZoneConverter/TimeZoneConverter.jpg

Introduction

Recently we had a requirement which demands knowledge of all the time zones and doing conversions from universal time to local time and vice versa. I searched for the code which would do this, but most of them happened to be in C#, article by Mike Dimmick is quite good but this was also in C# and there was very limited information available for doing this in C++. So I thought of putting together C++ code which does this on CodeProject. The different steps involved in writing this code are explained in the following sections. (I am not an English expert, but I tried my best to make the intention as clear as possible).

Enumerating TimeZones

There are no direct API's in Windows to enumerate time zones. We need to do get all the time zone information from the registry. The following key in the registry contains a list of time zones in alphabetical order in the form of sub keys:

"SOFTWARE\Microsoft\Windows NT\CurrentVersion\Time Zones"

Each subkey under the above subkey will give information about individual timezone. This individual subkey will have the following value names.

For eg: subkey ALASKAN STANDARD TIME will have information like this:

Display : (GMT - 9:00) Alaska
Dlt : Alaskan DayLight Time
Index : 0x00000003 (3)
MapID : 30,31
Std : Alaskan Standard Time
TZI : TimeZone Information in Binary Form

Of all these values, TZI happens to be critical information, which needs to be retrieved.
This value has the following data:

long Bias
long StandardBias
long DaylightBias
SYSTEMTIME StandardDate
SYSTEMTIME DaylightDate

We need to retrieve this information, first by enumerating all sub keys under "SOFTWARE\Microsoft\Windows NT\CurrentVersion\Time Zones", then by querying values under each of the sub keys.

CTimeZoneInfoManager::EnumerateTimeZones() will have related code which would read all sub keys information of "SOFTWARE\Microsoft\Windows NT\CurrentVersion\Time Zones".

int CTimeZoneInfoManager::GetFullTimeZoneInfoFromRegistry() will have code which will fill the necessary time zone information structures, which are required for conversion.

Finally all the time zone information is collected in the template based class CArray<CRegTimeZoneInfo*, CRegTimeZoneInfo*> in alphabetical order.

Sorting TimeZones

Now we have time zone information in alphabetical form, but we need to have this information sorted on the offset from GMT time (bias). There are several ways to do this, but I have selected qsort() function. We need to supply the starting address of the array and a callback comparison function which will get called when sorting the CArray.

We need to pass the address of the first element of CArray and a comparison function to qsort(). The comparison function will decide the criteria on which we will sort the code. We need to sort time zones based on the offset from GMT or standard bias.

int CTimeZoneInfoManager::SortTimeZoneList()
TimeZoneComparer(const void *i_TZ1, const void *i_TZ2)

will have the related code.

FINALLY...THE TIME CONVERSION

The following APIs do the necessary conversion from local time in given TimeZone to universal time.

  • TzSpecificLocalTimeToSystemTime()
  • SystemTimeToTzSpecificLocalTime()

The SystemTimeToTzSpecificLocalTime() API takes TIME_ZONE_INFORMATION, Universal time as input and returns localtime which corresponds to supplied TIME_ZONE_INFORMATION. We will fill this TIME_ZONE_INFORMATION structure from the information we retrieved from the registry. One particular thing we need to be careful here is that the information we collected from registry for "StandardName" and "DayLightName" is in ANSI form, but TIME_ZONE_INFORMATION requires these values to be in widechar or Unicode format, so we need to use MultiByteToWideChar() and WideCharToMultiByte() functions to do the necessary conversions wherever necessary.

The TzSpecificLocalTimeToSystemTime() API does exactly the reverse of the above API, that is, it takes TIME_ZONE_INFORMATION and the corresponding local time as input and converts to universal time. But unfortunately, this is available only on XP and later OS versions!(to compile the code under XP, please uncomment "//#define _XP_OR_LATER" in TimeZoneInfoManager.cpp)

For the OS versions below XP, we can convert local time of currenttimezone to universal time using code like this:

int CTimeZoneInfoManager::SpecificLocalTimeToSystemTime
	(SYSTEMTIME* i_stLocal, SYSTEMTIME* o_stUniversal)
{

    FILETIME ft, ft_utc;

    if (!(SystemTimeToFileTime(i_stLocal, &ft) && 
          LocalFileTimeToFileTime(&ft, &ft_utc) &&
          FileTimeToSystemTime(&ft_utc,o_stUniversal)))
         {
            return 0;
         }

        return 1;
}

But SpecificLocalTimeToSystemTime() has other limitations too. It always considers daylight bias to be -60 and there may be a difference in behaviour of TzSpecificLocalTimeToSystemTime() and SpecificLocalTimeToSystemTime() in case of daylight savings.

The following functions have related code:

  • int CTimeZoneInfoManager::ConvertFromLocalToUTC()
  • int CTimeZoneInfoManager::ConvertFromUTClToLocal()

Sample Application

The application has very basic GUI. I have used simple datetimepicker control for I/O purposes of date and time. We can choose the option of converting UTCtoLocal or LocalToUTC from the provided radio buttons. Based on the selection, datepicker used for I/P gets enabled and datepicker used for O/P gets disabled. Whenever there is a change in the I/P, output changes automatically. We can select the required timezone from the combobox.

Conclusion

I hope this would help atleast some programmers who are looking for information related to timezones and timezone conversions.

History

  • 2nd January, 2006: Initial post

License

This article, along with any associated source code and files, is licensed under The Code Project Open License (CPOL)

About the Author

Ramesh Ketha

United States United States
No Biography provided

Comments and Discussions

 
GeneralRegarding Local Time to UTC conversion Pinmemberdhdfghdfhdfhg9-Sep-08 1:49 
AnswerRe: Regarding Local Time to UTC conversion Pinmemberprantlf4-Sep-09 2:14 
QuestionHow to make the TzSpecificLocalTimeToSystemTime work for Os &lt; Win XP Pinmembertrysunil13-Apr-08 16:24 
Hi Guys,
 
I saw the post and code that is being used. I think this code does not work properly under windows XP version. I have a new algorithm defined for fixing this problem.
 
This algorithm is defined for the results upto precision of nano-seconds.
 
Let me know if this works.. ENJOY
 
Please email me for details on trysunil@yahoo.com
 
Thanks and Regards,
Sunil
 

BOOL TzSpecificLocalTimeToSystemTime(LPTIME_ZONE_INFORMATION lpTimeZoneInformation, LPSYSTEMTIME lpLocalTime, LPSYSTEMTIME lpUniversalTime)
      {
          //variable defination section
            TIME_ZONE_INFORMATION tzinfo;
          _int64 Bias = 0;
          SYSTEMTIME lpTempUTime, lpTempLTime ;
          FILETIME ft, ft_utc;
            ULARGE_INTEGER t;
 
          // get the present timezone information
          if(::GetTimeZoneInformation(&tzinfo) == TIME_ZONE_ID_UNKNOWN)
          {
               //if timezone returns 0 then return false.
               return false;
          }
          //conver the local time to the UTC time using current timezone.
          if (!(SystemTimeToFileTime(lpLocalTime, &ft) &&
                  LocalFileTimeToFileTime(&ft, &ft_utc) &&
                  FileTimeToSystemTime(&ft_utc,lpUniversalTime)))
          {
               return false;
          }
 
          //copy the utc time to the temp utc time variable.
          lpTempUTime.wDay               = lpUniversalTime->wDay;
          lpTempUTime.wDayOfWeek          = lpUniversalTime->wDayOfWeek;
          lpTempUTime.wHour               = lpUniversalTime->wHour;
          lpTempUTime.wMilliseconds     = lpUniversalTime->wMilliseconds;
          lpTempUTime.wMinute               = lpUniversalTime->wMinute;
          lpTempUTime.wMonth               = lpUniversalTime->wMonth;
          lpTempUTime.wSecond               = lpUniversalTime->wSecond;
          lpTempUTime.wYear               = lpUniversalTime->wYear;
 
          // convert the UTC Time to the local time using timezone properties.
          // store the local time in the temp local time variable.
          if(::SystemTimeToTzSpecificLocalTime(&tzinfo,&lpTempUTime,&lpTempLTime) == 0)
               return false;
 
          //find the offset between the temp local time and the local time which is
          //the input to this function. the output to this function in is the difference
          // in nano seconds.
          Bias = CompareTime(&lpTempLTime,lpLocalTime);
    
          // Actual conversion starts here.
          //   convert the systemtime to file time then add the offset to it
          // and convert the file time to the utc time.
            if (!::SystemTimeToFileTime(lpLocalTime, &ft))
               return FALSE;
          //convert the file time tot he Large integer type
            t.HighPart = ft.dwHighDateTime;
            t.LowPart   = ft.dwLowDateTime;
          //add the bias in nono seconds
          t.QuadPart += (LONGLONG(tzinfo.Bias) * LONGLONG(600000000));
          //add daylight bias in nano seconds
            t.QuadPart += Bias;
           
          //convert the large integer back to filetime
          ft.dwLowDateTime   = t.LowPart;
            ft.dwHighDateTime = t.HighPart;
            //convert it back to the system time and return the result of function.
          return ::FileTimeToSystemTime(&ft, lpUniversalTime);
         
      }
 
      _int64 CompareTime(LPSYSTEMTIME lpTempLTime,LPSYSTEMTIME lpLocalTime)
     {
          // variable declaration area
          FILETIME ftTempLTime,ftLocalTime;
          ULARGE_INTEGER LiTempLTime, LiLocalTime     ;
          // convert the systemtime to file time
          SystemTimeToFileTime(lpTempLTime, &ftTempLTime);
          SystemTimeToFileTime(lpLocalTime, &ftLocalTime);
          // convert the file time to the large integer type
          LiTempLTime.HighPart = ftTempLTime.dwHighDateTime;
          LiTempLTime.LowPart   = ftTempLTime.dwLowDateTime;
          LiLocalTime.HighPart = ftLocalTime.dwHighDateTime;
          LiLocalTime.LowPart   = ftLocalTime.dwLowDateTime;
          // return the difference in nano-seconds
          return (LiLocalTime.QuadPart - LiTempLTime.QuadPart);
    
     }
AnswerRe: How to make the TzSpecificLocalTimeToSystemTime work for Os &lt; Win XP Pinmembertrysunil13-Apr-08 18:54 
GeneralRe: How to make the TzSpecificLocalTimeToSystemTime work for Os &lt; Win XP PinmemberAnatoly Ivasyuk29-Aug-08 10:56 
GeneralRe: How to make the TzSpecificLocalTimeToSystemTime work for Os &lt; Win XP PinmemberHugh Jizak16-Dec-08 10:17 
GeneralRe: How to make the TzSpecificLocalTimeToSystemTime work for Os &lt; Win XP Pinmemberehsiung4-Jan-09 13:17 
GeneralRe: How to make the TzSpecificLocalTimeToSystemTime work for Os &lt; Win XP Pinmembertrysunil4-Jan-09 13:33 
QuestionRe: How to make the TzSpecificLocalTimeToSystemTime work for Os &lt; Win XP PinmemberZhi Chen9-Feb-09 6:06 
AnswerRe: How to make the TzSpecificLocalTimeToSystemTime work for Os &lt; Win XP Pinmembertrysunil9-Feb-09 14:13 
QuestionRe: How to make the TzSpecificLocalTimeToSystemTime work for Os &lt; Win XP PinmemberZhi Chen13-Feb-09 10:24 
AnswerRe: How to make the TzSpecificLocalTimeToSystemTime work for Os &lt; Win XP Pinmemberehsiung12-Jun-09 7:11 
GeneralInvalid Property Value Pinmemberwhoisthis4-Jan-06 7:40 
GeneralRe: Invalid Property Value PinmemberBrian (BH)10-Jan-06 23:15 
GeneralRe: Invalid Property Value Pinmembershadow7919-Jul-06 20:25 
GeneralRe: Invalid Property Value PinmembermacALEX28-Sep-06 19:02 
GeneralRe: Invalid Property Value Pinmemberp4abit24-Apr-07 0:49 
GeneralRe: Invalid Property Value PinmemberArunKumar Yalamarthy22-May-07 20:23 

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