I am writing a Linux (Ubuntu and Debian Lenny) application in C++.
Now I need to know the distance/offset between UTC and the currently set system time at a given day in the past. Since I need to convert recorded data, I the distance need to be calculated with respect to a date in the past (which may have a different DST setting than the current day).
Anyone know how to do this?
Edit: Reading the first answer I think I was misunderstood: I do not want to compare dates/times. I have date/time values which I want to convert from UTC to local time.
Prepare the tm structure with date:
struct tm date;
memset(&date,0,sizeof(date));
date.tm_year = year - 1900;
date.tm_mon = month - 1;
date.tm_mday = day;
date.tm_hour = hour;
date.tm_min = minute;
date.tm_sec = second;
date.tm_isdst = -1; // VERY IMPORTANT
mktime(&date); /// fill rest of fields
And then take a look on tm_gmtoff
printf("%d\n",date.tm_gmtoff);
This is distance from UTC.
Now this is Linux and BSD specific, it would not work on other stystems, and this works
with respect to DST.
Read man mktime for more information. And filling struct tm with correct values
P.S.: Converting from UTC to Local and back?
time_t posix_time = timegm(&UTC_Filled_struct_tm); // Linux specific line
localtime_r(&posix_time,&local_Filled_struct_tm);
Local to UTC
time_t posix_time = mktime(&local_Filled_struct_tm);
gmtime_r(&posix_time,&UTC_Filled_struct_tm);
I think you may benefit from using Boost.DateTime or ICU.
As for Boost.DateTime tt might be like this:
1) You prepare a database with timezone information Boost.Datetime and create a timezone. Timezones are important since they hold information about DST
tz_database tz_db;
tz_db.load_from_file("./date_time_zonespec.csv");
time_zone_ptr nyc = tz_db.time_zone_from_region("America/New_York"); // or other timezone
Or just create a timezone like this.
std::string kaliningrad_string = "EET+02:00:00EEST+01:00:00,M3.5.0/02:00:00,M10.5.0/03:00:00";
boost::local_time::time_zone_ptr kaliningrad_tzone_posix(new boost::local_time::posix_time_zone(kaliningrad_string));
std::string vladivostok_string = "VLAT+10:00:00VLAST+01:00:00,M3.5.0/02:00:00,M10.5.0/03:00:00";
boost::local_time::time_zone_ptr vladivostok_tzone_posix(new boost::local_time::posix_time_zone(vladivostok_string));
Creating timezones using a string specification of timezones looks more difficult but you can make use of it if you can't find a particular timezone in date_time_zonespec.csv.
For example Samara used to be in UTC+4 before March 2010 and now it is in UTC+3. date_time_zonespec.csv doesn't have history of changes, so in this situation it is necessary to create a timezone out of the string specification. However I recall that ICU seems to have timezones with this sort of history ICU TimeZone Classes:
Time zone data changes often in
response to governments around the
world changing their local rules and
the areas where they apply. The ICU
time zone data is updated for each
release, and the easiest way to stay
up to date may be to upgrade to the
latest ICU release, which also
provides bug fixes, code improvements
and additional features.
3) Make a localtime you need, for example
local_date_time tmp(boost::gregorian::date(2010, 3, 28), boost::posix_time::time_duration(1,59,0),nyc, boost::local_time::local_date_time::EXCEPTION_ON_ERROR);
4) And then calculate difference using functions utc_time and local_time
There is an example on that page:
ptime pt(date(2004,Nov,5),
hours(10));
time_zone_ptr zone(new posix_time_zone("MST-07"));
local_date_time az(pt, zone);
az.utc_time(); // 10am 2004-Nov-5
az.local_time(); // 3am 2004-Nov-5
5) Another example. The local time is the same but UTC is different
local_date_time tmp(boost::gregorian::date(2010, 3, 28), boost::posix_time::time_duration(1,59,0),kaliningrad_tzone_posix, boost::local_time::local_date_time::EXCEPTION_ON_ERROR);
std::cout << "As is: " << tmp << ", UTC: " << tmp.utc_time() << std::endl;
local_date_time tmp(boost::gregorian::date(2010, 3, 28), boost::posix_time::time_duration(1,59,0),vladivostok_tzone_posix, boost::local_time::local_date_time::EXCEPTION_ON_ERROR);
std::cout << "As is: " << tmp << ", UTC: " << tmp.utc_time() << std::endl;
Make sure all the times are converted to UTC (see mktime() for example). You then can use either difftime() or the timeval_subtract example function from the same link, depending on what structure the times are stored in.
Related
In Python or Java you can get the UTC offset (at present time) given the IANA name of the timezone ("America/Los_Angeles" for instance). See Get UTC offset from time zone name in python for example.
How can you do the same using C/C++ on Ubuntu 14.04?
EDIT: Preferably in a thread-safe way (no environment variables).
You alluded to this fact, but it's important to note that the offset between UTC and the time in a time zone is not necessarily constant. If the time zone performs daylight saving (summer) time adjustments, the offset will vary depending on the time of year.
One way to find the offset is to take the time you're interested in, hand it to the localtime() function, then look at the tm_gmtoff field. Do this with the TZ environment variable set to the zone name you're interested in. Here's an example that does so for the current time:
#include <time.h>
#include <stdio.h>
int main()
{
setenv("TZ", "America/Los_Angeles", 1);
time_t t = time(NULL);
struct tm *tmp = localtime(&t);
printf("%ld\n", tmp->tm_gmtoff);
}
At the moment this prints -25200, indicating that Los Angeles is 25200 seconds, or 420 minutes, or 7 hours west of Greenwich. But next week (actually tomorrow) the U.S goes off of DST, at which point this code will start printing -28800.
This isn't guaranteed to work, since the tm_gmtoff field is not portable. But I believe all versions of Linux will have it. (You might have to compile with -D_BSD_SOURCE or something, or refer to the field as __tm_gmtoff. But in my experience it tends to work by default, as plain tm_gmtoff.)
The other way is to go back and forth with gmtime and mktime, as described in Sam Varshavchik's answer.
Addendum: You asked about not using environment variables. There is a way, but unfortunately it's even less standard. There are BSD functions tzalloc and localtime_rz which do the job, but they do not exist on Linux. Here's how the code looks:
timezone_t tz = tzalloc("America/Los_Angeles");
if(tz == NULL) return 1;
time_t t = time(NULL);
struct tm tm, *tmp = localtime_rz(tz, &t, &tm);
printf("%ld\n", tmp->tm_gmtoff);
For me this prints -28800 (because PDT fell back to PST just a few minutes ago).
If you had it, you could also use localtime_rz along with mktime in Sam Varshavchik's answer. And of course Howard Hinnant's library is pretty obviously thread-safe, not requiring mucking with TZ at all.
EDIT (OP): The code for localtime_rz and tzalloc can be downloaded from https://www.iana.org/time-zones and works on Ubuntu 14.04.
You could use this free open source C++11/14 library to do it like this:
#include "chrono_io.h"
#include "tz.h"
#include <iostream>
int
main()
{
using namespace date;
using namespace std::chrono;
auto zt = make_zoned("America/Los_Angeles", system_clock::now());
std::cout << zt.get_info().offset << '\n';
}
This currently outputs:
-25200s
Or you could format it differently like this:
std::cout << make_time(zt.get_info().offset) << '\n';
which currently outputs:
-07:00:00
The factory function make_zoned creates a zoned_time using the IANA name "America/Los_Angeles" and the current time from std::chrono::system_clock. A zoned_time has a member getter to get the information about the timezone at that time. The information is a type called sys_info which contains all kinds of useful information, including the current UTC offset.
The UTC offset is stored as a std::chrono::seconds. The header "chrono_io.h" will format durations for streaming. Or the make_time utility can be used to format the duration into hh:mm:ss.
The program above is thread-safe. You don't have to worry about some other process changing TZ out from under you, or changing the current time zone of the computer in any other way. If you want information about the current time zone, that is available too, just use current_zone() in place of "America/Los_Angeles".
If you wanted to explore other times, that is just as easy. For example beginning at Nov/6/2016 at 2am local time:
auto zt = make_zoned("America/Los_Angeles", local_days{nov/6/2016} + 2h);
The output changes to:
-28800s
-08:00:00
More information about this library was presented at Cppcon 2016 and can be viewed here:
https://www.youtube.com/watch?v=Vwd3pduVGKY
Use gmtime(), first, to convert the current epoch time into UTC time, then use mktime() to recalculate the epoch time, then compare the result to the real epoch time.
gmtime() calculates the struct tm in UTC, while mktime() assumes that the struct tm represents the current local calendar time. So, by making this round-about calculation, you indirectly figure out the current timezone offset.
Note that mktime() can return an error if struct tm cannot be convert to epoch time, which will happen during certain transitions between standard time and alternate time. It's up to you to figure out what that means, in your case.
The recipe looks something like this:
time_t t = time(NULL);
struct tm *tmp = gmtime(&t);
time_t t2 = mktime(tmp);
int offset = t - t2;
See the documentation of these library functions for more information.
To use a specific time zone, either set the TZ environment variable, or you can try using localtime_rz as in Steve Summit's answer. As mentioned, beware that mktime can sometimes return -1 for unconvertible times.
Have a Unix timestamp and I need equivalents of localtime_s, but where I can pass in the timezone. Alternately I'm looking for a way to change the timezone programatically for the app (not the whole system), so that localtime_s returns the correct values.
Strangely enough, if I set TZ environment variable in a shell to GMT, and then launch my application, localtime_s returns values in GMT.
I've tried:
::SetEnvironmentVariable("TZ", "GMT");
tzset();
But it does not change the results of later calls to localtime_s
Note that I'm running the application on Windows 7, using VS 2013.
If you are willing to use a free, open-source 3rd party library, this works on VS-2013. There are many examples on how to use it here:
https://github.com/HowardHinnant/date/wiki/Examples-and-Recipes
For example here is how to output the current time in New York and Chicago:
#include "tz.h"
#include <iostream>
int
main()
{
auto now = std::chrono::system_clock::now();
std::cout << "Current time in New York: "
<< date::make_zoned("America/New_York", now) << '\n';
std::cout << "Current time in Chicago : "
<< date::make_zoned("America/Chicago", now) << '\n';
}
This just output for me:
Current time in New York: 2016-08-12 10:36:24.670966 EDT
Current time in Chicago : 2016-08-12 09:36:24.670966 CDT
I have been wracking my head crazy trying to figure this out with this API
My original implementation was something like:
// TimezonePtr is just a share_ptr to the timezone
std::tm getGMT(const std::tm& rawtime, TimezonePtr tz)
{
std::tm result = rawtime;
const auto loct = mktime_z(tz.get(), &result);
gmtime_r(&loct, &result);
return result;
}
However, this does not take into account DST. For example, if I feed it a date of Sep 28 2012 15:54:24 I get back Sep 28 2012 20:54:24, which is incorrect. It looks like I want to use localtime_rz, except that takes an epoch, which is driving me nuts because if I could get the epoch then I'd already have my answer. :(
How can I accomplish this?
mktime_z takes a struct tm as one of its arguments. If you don't know whether DST is in effect for the input date, you want to set the tm_isdst member of that tm to -1 to signify that the system should figure out whether DST is in effect for that date/time/timezone when you call mktime.
At least for me, this seems to work correctly (i.e., it correctly concludes that at least in my time zone, DST was in effect in September of 2012).
In addition to Jerry Coffin's correct (and up-voted) answer, I wanted to show how this computation could be done with a modern C++11/14 library (free and open source).
I've kept the API the same in the interest of making the code easy to compare:
template <class Duration>
auto
getGMT(date::local_time<Duration> rawtime, const date::time_zone* tz)
{
return tz->to_sys(rawtime);
}
This returns a std::chrono::time_point<system_clock, Duration> where Duration is the finer of the input Duration and seconds. If the ragtime doesn't have a unique mapping to UTC according to the indicated time zone, an exception will be thrown. Such an event can occur (for example) if rawtime is during a daylight saving transition and occurs twice, or not at all. If desired, there exists API for avoiding the exception if you want to "pre-decide" how you would like to map ambiguous and non-existent local times into UTC.
This function can be exercised like this:
#include "tz.h"
#include <iostream>
int
main()
{
using namespace date;
using namespace std::chrono_literals;
std::cout << getGMT(local_days{sep/28/2012} + 15h + 54min + 24s,
current_zone()) << " UTC\n";
std::cout << getGMT(local_days{sep/28/2012} + 15h + 54min + 24s,
locate_zone("America/New_York")) << " UTC\n";
}
This exercises the code twice:
With whatever the current time zone is for this computer.
With the time zone "America/New_York"
For me these are both the same time zone, resulting in the following output:
2012-09-28 19:54:24 UTC
2012-09-28 19:54:24 UTC
Not only is it simpler to use this library than the BSD ctime API,
but this API has type safety. For example the UTC time point and local time point are represented by different types, allowing the compiler to tell you if you accidentally use the wrong one. In contrast the BSD ctime API uses the same type (tm) for both local time and UTC.
I see a strange behavior while fetching the modification time of a file.
we have been calling _stat64 method to fetch the file modification in our project as following.
int my_win_stat( const char *path, struct _stati64 *buf)
{
if(_stati64( path, buf) == 0)
{
std::cout<<buf->st_mtime << std::endl; //I added to ensure if value is not changing elsewhere in the function.
}
...........
...........
}
When I convert the epoch time returned by st_mtime variable using epoch convertor, it shows 2:30 hrs ahead of current time set on my system.
When I call same API as following from different test project, I see the correct mtime (i.e. according to mtime of file shown by my system).
if (_stat64("D:\\engine_cost.frm", &buffer) == 0)
std::cout << buffer.st_mtime << std::endl;
Even I called GetFileTime() and converted FILETIME to epoch time with the help of this post. I get the correct time according to time set the system.
if (GetFileTime(hFile, &ftCreate, &ftAccess, &ftWrite))
{
ULARGE_INTEGER ull;
ull.LowPart = ftWrite.dwLowDateTime;
ull.HighPart = ftWrite.dwHighDateTime;
std::cout << (ull.QuadPart / 10000000ULL - 11644473600ULL);
}
What I am not able to figure out is why does the time mtime differ when called through my existing project?
What are the parameters that could affect the output of mtime ?
What else I could try to debug the problem further ?
Note
In VS2013, _stati64 is a macro which is replaced replaced by _stat64.
File system is NTFS on windows 7.
Unix time is really easy to deal with. It's the number of seconds since Jan 1, 1970 (i.e. 0 represents that specific date).
Now, what you are saying is that you are testing your time (mtime) with a 3rd party tool in your browser and expects that to give you the right answer. So... let's do a test, the following number is Sept 1, 2015 at 00:00:00 GMT.
1441065600
If you go to Epoch Converter and use that very value, does it give you the correct GMT? (if not, something is really wrong.) Then look at the local time value and see whether you get what you would expect for GMT midnight. (Note that I use GMT since Epoch Converter uses that abbreviation, but the correct abbreviation is UTC.)
It seems to me that it is very likely that your code extracts the correct time, but the epoch convertor fails on your computer for local time.
Note that you could just test in your C++ program with something like this:
std::cerr << ctime(&buf->st_mtime) << std::endl;
(just watch out as ctime() is not thread safe)
That will give you the data according to your locale on your computer at runtime.
To better control the date format, use strftime(). That function makes use of a tm structure so you have to first call gmtime or localtime.
An year later I ran into the similar problem but scenario is little different. But this time I understand why there was a +2:30Hrs of gap. I execute the C++ program through a perl script which intern sets the timezone 'GMT-3' and my machine had been in timezone 'GMT+5:30'. As a result there was a difference of '2:30Hrs'.
Why ? As Harry mentioned in this post
changing the timezone in Perl is probably causing the TZ environment variable to be set, which affects the C runtime library as per the documentation for _tzset.
[Updated]
I am trying to find to correct way to convert a timestamp in UTC to different timezones.
The exact problem: I have timestamps in my program and its always stored in UTC so its fine. however, i have to be able to display them (actually write them to files) in different timezones according to user preferences.
I am working on linux but I would like to write pateform-independant code.
I don't want to use boost libraries (we are already using Qt which does not provide as much functions as boost for dates).
I would like to write thread-safe code
I would like to identify the timezones like US/Eastern to simplify the configuration (it is done by users and im not very confident that they would make correct use of the abbreviations like EST, CET, CEST...).
I already looked on Internet and found some more or less working code but
most of the time it uses the TZ env variable which is said to be a not thread-safe method.
It uses abbreviations for the timezone (like EST, CET...).
Could anybody indicate me a good approach?
Here is what I have now (found on Internet some days ago and modified to used my Qt library in this example).
This code is probably not thread-safe.
NEW VERSION:
Still not thread-safe but it more or less do the job.
Probably not easily portable to window environment.
It handles the daylight change
see bellow example (daylight change in Paris happens on 25 Mars 2012 at 01H00 UTC (passing from 02H00 localtime at 03H00 localtime).
It is an example to convert Timestamp from UTC to Paris (has daylight change) & Kuala_Lumpur (does not have daylight change).
#include <QtCore/QCoreApplication>
#include <QDateTime>
#include <stdio.h>
#include <stdlib.h>
#include <QDebug>
void treatTimestamp(QString timestamp,QString format);
int main(int argc, char *argv[])
{
QString format = "MM:dd:yyyy hh:mm:ss";
treatTimestamp("03:25:2012 00:59:59",format);
qDebug()<<"---------------------";
treatTimestamp("03:25:2012 01:00:00",format);
return 0;
}
void treatTimestamp(QString timestamp_s,QString format)
{
unsetenv("TZ");
setenv("TZ", "UTC", 1);
QDateTime timestamp = QDateTime::fromString(timestamp_s, format);
qDebug()<<"CUSTOM TS UTC:"<<timestamp.toUTC().toString(format).toStdString().c_str();;
time_t tmp = timestamp.toUTC().toTime_t();
setenv("TZ", ":Asia/Kuala_Lumpur", 1);
qDebug()<<"CUSTOM TS KL:"<<QDateTime::fromTime_t(tmp).toString(format);
setenv("TZ", "Europe/Paris", 1);
qDebug()<<"CUSTOM TS Paris:"<<QDateTime::fromTime_t(tmp).toString(format);
unsetenv("TZ");
}
Output (First : one second before the timechange, Second: one second after).
CUSTOM TS LOC: 03:25:2012 01:00:00
CUSTOM TS UTC: 03:25:2012 01:00:00
CUSTOM TS KL: "03:25:2012 09:00:00"
CUSTOM TS Paris: "03:25:2012 03:00:00"
CUSTOM TS LOC: 03:25:2012 03:00:00
CUSTOM TS UTC: 03:25:2012 03:00:00
CUSTOM TS KL: "03:25:2012 11:00:00"
CUSTOM TS Paris: "03:25:2012 05:00:00"
According to this thread, using QDateTime it is possible to do dateTime.addSecs(3600*timeZoneOffset); where dateTime is QDateTime.
According to gmtime reference, there's no built-in timezone support in C library, but you can "kinda" simulate them by adding requiring offset to tm->tm_hour. Which won't adjust date correctly (unlike QDateTime method), by the way.
According to mktime reference, mktime will "normalize" datetime values, so you could add time offset to tm_hour, call mktime. However, it isn't specified HOW mktime adjusts fileds of struct tm - if you say, set tm_hour to 27, will it clamp tm_hour to 23 or will set tm_hour to 3, increasing tm_day (and possibly month/year)?
If I were you, I'd simply use QDateTime::addSecs method.
I give +1 to SigTerm as his answers have been constructive.
In the end i validate with users that one running process will only need 2 timezones : a specified one and the UTC one (mostly for logging).
So in the end I use this at the top of the program
unsetenv("TZ");
setenv("TZ", "", 1);
Then in the specific parts where i need UTC time i always call the Qt toUTC method.
It is really not satisfying but the full software is about data acquisition and the timestamp is an important part of it so i didn't want to make my own calculation in the code.
I heard that Qt5 will have an implementation of timezone manipulation similar to what exists in the boost library. Maybe ill refactor the code when it will be out,.