C++ std::localtime daylight saving time rule (dst) european vs american - c++

I've problem with use of the std::localtime function. When I transform a std::time_t to a local struct tm, it always use the american daylight saving time whereas I want to use the european one (France).
UTC : 03/19/16 16:56 is transformed in Local : 03/19/16 18:56.
At this date, normally, local is 17:56 (UTC+1). The DST happens on the 27th in France.
After several tests, it seems that the used DST is based on the american rule : DST happens on second sunday in march.
I've also changed the TZ environement variable, but it also fails.
`
if( putenv( "TZ=CET-1CEST-2,M3.5.0/2,M10.5.0/3" ) != 0 ) {
std::cout << "Unable to set TZ" << std::endl;
} else {
tz = getenv("TZ");
if (tz) std::cout << tz << std::endl;
else std::cout << "TZ not defined" << std::endl; tzset();
}
struct std::tm t;
t.tm_sec = 0;
t.tm_min = 56;
t.tm_hour = 18;
t.tm_mday = 19;
t.tm_mon = 3-1;
t.tm_year = 2016-1900;
t.tm_isdst = -1;
std::time_t tt = std::mktime(&t);
std::cout << "UTC: " << std::put_time(std::gmtime(&tt), "%c %Z") << '\n'; // UTC : 03/19/16 16:56
std::cout << "local: " << std::put_time(std::localtime(&tt), "%c %Z") << '\n'; // Local : 03/19/16 18:56 (not waited result)
`
As a precision, I use the bcc32c compiler (the embarcadero C++ clang based computer).
I hope I'm clear enough and you'll be able to help me.
Thanks in advance

If you have C++11 (or later) available, which includes <chrono>, and if you are willing to work with the <chrono> system. And if you are willing to use this free, open-source timezone library. Then this is a very simple matter:
#include "tz.h"
#include <iostream>
int
main()
{
using namespace date;
using namespace std::chrono_literals;
auto zt = make_zoned("Europe/Paris", sys_days{2016_y/mar/19} + 18h + 56min);
std::cout << format("%m/%d/%y %H:%M %Z", zt.get_sys_time()) << '\n';
std::cout << format("%m/%d/%y %H:%M %Z", zt) << '\n';
}
Output:
03/19/16 18:56 UTC
03/19/16 19:56 CET
In short, you form one zoned_time by pairing a timezone of your choosing
("Europe/Paris"), with a sys_time (UTC). Then you can format that zoned_time both by extracting the sys_time back out of it, and by formatting the zoned_time itself, which will use the local_time of the zoned_time.
I attempted to use formatting that was consistent with your comments. You could of course you any format you want. You can also include any std::locale your system supports as the first argument in the calls to format (which %c would take advantage of).
You can also extract individual fields both from the sys_time and from the local_time if you need to, or do other date or time computations. This is more than just a formatting library.
If you are starting with a time_t, there is a static member function of std::chrono::system_clock to convert a time_t to a system_clock::time_point. You can then use that time_point in place of sys_days{2016_y/mar/19} + 18h + 56min in this example:
auto zt = make_zoned("Europe/Paris", system_clock::from_time_t(t));

Related

From xs:dateTime to std::chrono::timepoint [duplicate]

Consider a historic date string of format:
Thu Jan 9 12:35:34 2014
I want to parse such a string into some kind of C++ date representation, then calculate the amount of time that has passed since then.
From the resulting duration I need access to the numbers of seconds, minutes, hours and days.
Can this be done with the new C++11 std::chrono namespace? If not, how should I go about this today?
I'm using g++-4.8.1 though presumably an answer should just target the C++11 spec.
std::tm tm = {};
std::stringstream ss("Jan 9 2014 12:35:34");
ss >> std::get_time(&tm, "%b %d %Y %H:%M:%S");
auto tp = std::chrono::system_clock::from_time_t(std::mktime(&tm));
GCC prior to version 5 doesn't implement std::get_time. You should also be able to write:
std::tm tm = {};
strptime("Thu Jan 9 2014 12:35:34", "%a %b %d %Y %H:%M:%S", &tm);
auto tp = std::chrono::system_clock::from_time_t(std::mktime(&tm));
New answer for old question. Rationale for the new answer: The question was edited from its original form because tools at the time would not handle exactly what was being asked. And the resulting accepted answer gives a subtly different behavior than what the original question asked for.
I'm not trying to put down the accepted answer. It's a good answer. It's just that the C API is so confusing that it is inevitable that mistakes like this will happen.
The original question was to parse "Thu, 9 Jan 2014 12:35:34 +0000". So clearly the intent was to parse a timestamp representing a UTC time. But strptime (which isn't standard C or C++, but is POSIX) does not parse the trailing UTC offset indicating this is a UTC timestamp (it will format it with %z, but not parse it).
The question was then edited to ask about "Thu Jan 9 12:35:34 2014". But the question was not edited to clarify if this was a UTC timestamp, or a timestamp in the computer's current local timezone. The accepted answer implicitly assumes the timestamp represents the computer's current local timezone because of the use of std::mktime.
std::mktime not only transforms the field type tm to the serial type time_t, it also performs an offset adjustment from the computer's local time zone to UTC.
But what if we want to parse a UTC timestamp as the original (unedited) question asked?
That can be done today using this newer, free open-source library.
#include "date/date.h"
#include <iostream>
#include <sstream>
int
main()
{
using namespace std;
using namespace date;
istringstream in{"Thu, 9 Jan 2014 12:35:34 +0000"};
sys_seconds tp;
in >> parse("%a, %d %b %Y %T %z", tp);
}
This library can parse %z. And date::sys_seconds is just a typedef for:
std::chrono::time_point<std::chrono::system_clock, std::chrono::seconds>
The question also asks:
From the resulting duration I need access to the numbers of seconds, minutes, hours and days.
That part has remained unanswered. Here's how you do it with this library.
#include "date/date.h"
#include <chrono>
#include <iostream>
#include <sstream>
int
main()
{
using namespace std;
using namespace date;
istringstream in{"Thu, 9 Jan 2014 12:35:34 +0000"};
sys_seconds tp;
in >> parse("%a, %d %b %Y %T %z", tp);
auto tp_days = floor<days>(tp);
auto hms = hh_mm_ss<seconds>{tp - tp_days};
std::cout << "Number of days = " << tp_days.time_since_epoch() << '\n';
std::cout << "Number of hours = " << hms.hours() << '\n';
std::cout << "Number of minutes = " << hms.minutes() << '\n';
std::cout << "Number of seconds = " << hms.seconds() << '\n';
}
floor<days> truncates the seconds-precision time_point to a days-precision time_point. If you subtract the days-precision time_point from tp, you're left with a duration that represents the time since midnight (UTC).
The type hh_mm_ss<seconds> takes any duration convertible to seconds (in this case time since midnight) and creates a {hours, minutes, seconds} field type with getters for each field. If the duration has precision finer than seconds this field type will also have a getter for the subseconds. Prior to C++17, one has to specify that finer duration as the template parameter. In C++17 and later it can be deduced:
auto hms = hh_mm_ss{tp - tp_days};
Finally, one can just print out all of these durations. This example outputs:
Number of days = 16079d
Number of hours = 12h
Number of minutes = 35min
Number of seconds = 34s
So 2014-01-09 is 16079 days after 1970-01-01.
Here is the full example but at milliseconds precision:
#include "date/date.h"
#include <chrono>
#include <iostream>
#include <sstream>
int
main()
{
using namespace std;
using namespace std::chrono;
using namespace date;
istringstream in{"Thu, 9 Jan 2014 12:35:34.123 +0000"};
sys_time<milliseconds> tp;
in >> parse("%a, %d %b %Y %T %z", tp);
auto tp_days = floor<days>(tp);
hh_mm_ss hms{tp - tp_days};
std::cout << tp << '\n';
std::cout << "Number of days = " << tp_days.time_since_epoch() << '\n';
std::cout << "Number of hours = " << hms.hours() << '\n';
std::cout << "Number of minutes = " << hms.minutes() << '\n';
std::cout << "Number of seconds = " << hms.seconds() << '\n';
std::cout << "Number of milliseconds = " << hms.subseconds() << '\n';
}
Output:
2014-01-09 12:35:34.123
Number of days = 16079d
Number of hours = 12h
Number of minutes = 35min
Number of seconds = 34s
Number of milliseconds = 123ms
This library is now part of C++20, but is in namespace std::chrono and found in the header <chrono>.
This is rather C-ish and not as elegant of a solution as Simple's answer, but I think it might work. This answer is probably wrong but I'll leave it up so someone can post corrections.
#include <iostream>
#include <ctime>
int main ()
{
struct tm timeinfo;
std::string buffer = "Thu, 9 Jan 2014 12:35:00";
if (!strptime(buffer.c_str(), "%a, %d %b %Y %T", &timeinfo))
std::cout << "Error.";
time_t now;
struct tm timeinfo2;
time(&now);
timeinfo2 = *gmtime(&now);
time_t seconds = difftime(mktime(&timeinfo2), mktime(&timeinfo));
time(&seconds);
struct tm result;
result = *gmtime ( &seconds );
std::cout << result.tm_sec << " " << result.tm_min << " "
<< result.tm_hour << " " << result.tm_mday;
return 0;
}
Cases covered (code is below):
since a give date until now
long int min0 = getMinutesSince( "2005-02-19 12:35:00" );
since the epoch until now
long int min1 = getMinutesSince1970( );
between two date+hours (since the epoch until a given date)
long int min0 = getMinutesSince1970Until( "2019-01-18 14:23:00" );
long int min1 = getMinutesSince1970Until( "2019-01-18 14:27:00" );
cout << min1 - min0 << endl;
Complete code:
#include <iostream>
#include <chrono>
#include <sstream>
#include <string>
#include <iomanip>
using namespace std;
// ------------------------------------------------
// ------------------------------------------------
long int getMinutesSince1970Until( string dateAndHour ) {
tm tm = {};
stringstream ss( dateAndHour );
ss >> get_time(&tm, "%Y-%m-%d %H:%M:%S");
chrono::system_clock::time_point tp = chrono::system_clock::from_time_t(mktime(&tm));
return
chrono::duration_cast<chrono::minutes>(
tp.time_since_epoch()).count();
} // ()
// ------------------------------------------------
// ------------------------------------------------
long int getMinutesSince1970() {
chrono::system_clock::time_point now = chrono::system_clock::now();
return
chrono::duration_cast<chrono::minutes>( now.time_since_epoch() ).count();
} // ()
// ------------------------------------------------
// ------------------------------------------------
long int getMinutesSince( string dateAndHour ) {
tm tm = {};
stringstream ss( dateAndHour );
ss >> get_time(&tm, "%Y-%m-%d %H:%M:%S");
chrono::system_clock::time_point then =
chrono::system_clock::from_time_t(mktime(&tm));
chrono::system_clock::time_point now = chrono::system_clock::now();
return
chrono::duration_cast<chrono::minutes>(
now.time_since_epoch()-
then.time_since_epoch()
).count();
} // ()
// ------------------------------------------------
// ------------------------------------------------
int main () {
long int min = getMinutesSince1970Until( "1970-01-01 01:01:00" );
cout << min << endl;
long int min0 = getMinutesSince1970Until( "2019-01-18 14:23:00" );
long int min1 = getMinutesSince1970Until( "2019-01-18 14:27:00" );
if ( (min1 - min0) != 4 ) {
cout << " something is wrong " << endl;
} else {
cout << " it appears to work !" << endl;
}
min0 = getMinutesSince( "1970-01-01 01:00:00" );
min1 = getMinutesSince1970( );
if ( (min1 - min0) != 0 ) {
cout << " something is wrong " << endl;
} else {
cout << " it appears to work !" << endl;
}
} // ()

Get year/month/day h/m/s/ns from Boost local_date_time object?

I have converted a timestamp (UTC) since Epoch to a boost::posix_time::ptime and applied a timezone, resulting in a boost::local_time::local_date_time object. However, I cannot retrieve the year, month, day, hh/mm/ss/ns or nanoseconds since midnight for the adjusted date time.
It appears the local_date_time object doesn't provide getters for these.
I can't get these from the boost::posix_time::ptime because it hasn't been shifted for timezone.
What's the best approach?
using namespace boost::gregorian;
using namespace boost::local_time;
using namespace boost::posix_time;
// Create timezone
tz_database tz_db;
tz_db.load_from_file("libs/date_time/data/date_time_zonespec.csv");
time_zone_ptr chicago_tz = tz_db.time_zone_from_region("America/Chicago");
// Create Epoch offset (seconds)
std::time_t btime_ = nanosSinceEpochUTC / 1E9;
ptime dateTime = boost::posix_time::from_time_t(btime_);
// Create local Chicago time at Epoch offset
const local_date_time chicago(dateTime, chicago_tz);
// I need to retrieve the year/month/day/hh/mm/ss etc from the adjusted time, not the ptime.
The time properties are in the time_of_day() sub object. The representation type of that subobject is time_duration and it has all the accessors you want:
Live On Coliru
#include <boost/date_time/local_time/local_time.hpp>
#include <boost/date_time/tz_db_base.hpp>
int main() {
// Create timezone
boost::local_time::tz_database tz_db;
{
std::istringstream iss(R"("America/Chicago","CST","Central Standard Time","CDT","Central Daylight Time","-06:00:00","+01:00:00","2;0;3","+02:00:00","1;0;11","+02:00:00")");
tz_db.load_from_stream(iss);
}
auto chicago_tz = tz_db.time_zone_from_region("America/Chicago");
// Create Epoch offset (seconds)
std::time_t btime_ = 1596241091; // nanosSinceEpochUTC / 1E9;
auto dateTime = boost::posix_time::from_time_t(btime_);
std::cout << "ptime: " << dateTime << "\n";
// Create local Chicago time at Epoch offset
const boost::local_time::local_date_time chicago(dateTime, chicago_tz);
std::cout << "local_date_time: " << chicago << "\n";
// I need to retrieve the year/month/day/hh/mm/ss etc from the adjusted
// time, not the ptime.
std::cout << "year/m/d: " << chicago.local_time().date() << "\n";
auto tod = chicago.local_time().time_of_day();
std::cout << "time_of_day: " << tod << "\n";
std::cout << "hh, mm, ss: " <<
tod.hours() << ", " <<
tod.minutes() << ", " <<
tod.seconds() << "\n";
}
As you can see, it's important to access through the local_time() accessor. Note how all the items (day, month, hours) wrapped back correctly according to the time zone.
Prints:
ptime: 2020-Aug-01 00:18:11
local_date_time: 2020-Jul-31 19:18:11 CDT
year/m/d: 2020-Jul-31
time_of_day: 19:18:11
hh, mm, ss: 19, 18, 11
What I do is as follows: I am naming one class after the name of a time zone. In my case gmt and fetch everything via instances of my very own time as well as date class.
class gmt {
string get(string& s) {
this->dt= this->d.getDay();
this->tm= this->d.getTime();
s= "expires="+this->d.getName(this->d.getDayName())
+", " +this->dt.substr( 0, 2) +"-" +this->d.getMonthName() +"-" +this->dt.substr( 8, 2)
+" " +this->tm +" GMT";
return s;
}
friend class cookie;
string dt, tm;
MyDate d;
};
Okay, it isn't "boost" but you can solve it via its own std and isn't that contrary.

time_t to boost date conversion giving incorrect result

I have a collection of unix timestamps I am converting to boost (1.65.1) dates but the conversions seem to break down when they get too far in the future. Anything around 2040 and beyond seems to be wrapping in some way back to post 1900.
Given the following code...
{
std::time_t t = 1558220400;
boost::gregorian::date date = boost::posix_time::from_time_t(t).date();
std::cout << "Date: " << date << std::endl;
}
{
std::time_t t = 2145500000;
boost::gregorian::date date = boost::posix_time::from_time_t(t).date();
std::cout << "Date: " << date << std::endl;
}
{
std::time_t t = 2500000000;
boost::gregorian::date date = boost::posix_time::from_time_t(t).date();
std::cout << "Date: " << date << std::endl;
}
... I get the following output...
Date: 2019-May-18
Date: 2037-Dec-27
Date: 1913-Feb-13
... however I am expecting the following output...
Expected output:
Date: 2019-May-18
Date: 2037-Dec-27
Date: 2049-Mar-22
Is there something I am doing wrong here?
It appears that you're experiencing the Year 2038 problem.
The largest number representable by 32 bit signed integer is 2'147'483'647. 2'147'483'647 seconds since 00:00:00 UTC on 1st of January 1970 (the UNIX epoch) is 03:14:07 UTC on 19th of January 2038. Any UNIX time after that is unrepresentable using a 32 bit signed integer.
Either std::time_t on the system is 32 bits, or it is converted into 32 bits inside the boost library. You can see from the source that boost converts the input into long using static_cast (and still does in version 1.70). long is 32 bits for example on windows, even on 64 bit architectures. It is 64 bits on many other systems such as 64 bit Linux.
In C++20 this can now look like:
#include <chrono>
#include <iostream>
int
main()
{
using namespace std::chrono;
{
std::time_t t = 1558220400;
auto date = floor<days>(system_clock::from_time_t(t));
std::cout << "Date: " << date << '\n';
}
{
std::time_t t = 2145500000;
auto date = floor<days>(system_clock::from_time_t(t));
std::cout << "Date: " << date << '\n';
}
{
std::time_t t = 2500000000;
auto date = floor<days>(system_clock::from_time_t(t));
std::cout << "Date: " << date << '\n';
}
}
Output:
Date: 2019-05-18
Date: 2037-12-27
Date: 2049-03-22
If your time_t is 32 bits, then the above isn't quite sufficient to fix the problem. In that case, you must avoid the C API completely. This looks like:
{
auto t = 1558220400;
auto date = floor<days>(sys_seconds{seconds{t}});
std::cout << "Date: " << date << '\n';
}
{
auto t = 2145500000;
auto date = floor<days>(sys_seconds{seconds{t}});
std::cout << "Date: " << date << '\n';
}
{
auto t = 2500000000;
auto date = floor<days>(sys_seconds{seconds{t}});
std::cout << "Date: " << date << '\n';
}
If your vendor isn't shipping this part of C++20 yet, a free, open-source preview that works with C++11/14/17 is available.1
Just add:
#include "date/date.h"
...
using namespace date;
1 Full disclosure: I am the lead author of this library. I am not pursuing any financial gain from this effort. But sometimes people get grumpy if I don't fully disclose this information.
As noted by eerorika this is caused by integer overflow since the boost::posix_time::from_time_t is casting the 64bit time_t value to a 32 bit long (on Windows).
If you are in a pinch and find yourself in the same position then you can use the following function to perform the conversion:
boost::gregorian::datetimet_to_date(time_t t)
{
auto time = boost::posix_time::ptime(boost::gregorian::date(1970,1,1));
int64_t current_t = t;
long long_max = std::numeric_limits<long>::max();
while(current_t > 0)
{
long seconds_to_add = 0;
if(current_t >= long_max)
seconds_to_add = long_max;
else
seconds_to_add = static_cast<long>(current_t);
current_t -= seconds_to_add;
time += boost::posix_time::seconds(seconds_to_add);
}
return time.date();
}

Adding and subtracting time with tm

So say I set a time in tm to be 23:00:00
ptm->tm_hour = 23; ptm->tm_min = 0; ptm->tm_sec = 0;
And I want to allow a user to subtract time from this
ptm->tm_hour -= hourinput; ptm->tm_min -= minuteinput; ptm->tm_sec -= secondinput;
If the user subtracts 0 hours, 5 minutes, and 5 seconds, instead of showing up as 22:54:55, it will show up as 23:-5:-5.
I suppose I could do a bunch of if statements to check if ptm is below 0 and account for this, but is there a more efficient way of getting the proper time?
Yes, you can use std::mktime for this. It doesn't just convert a std::tm to a std::time_t, it also fixes the tm if some field went out of range. Consider this example where we take the current time and add 1000 seconds.
#include <iostream>
#include <iomanip> // put_time
#include <ctime>
int main(int argc, char **argv) {
std::time_t t = std::time(nullptr);
std::tm tm = *std::localtime(&t);
std::cout << "Time: " << std::put_time(&tm, "%c %Z") << std::endl;
tm.tm_sec += 1000; // the seconds are now out of range
//std::cout << "Time in 1000 sec" << std::put_time(&tm, "%c %Z") << std::endl; this would crash!
std::mktime(&tm); // also returns a time_t, but we don't need that here
std::cout << "Time in 1000 sec: " << std::put_time(&tm, "%c %Z") << std::endl;
return 0;
}
My Output:
Time: 01/24/19 09:26:46 W. Europe Standard Time
Time in 1000 sec: 01/24/19 09:43:26 W. Europe Standard Time
As you can see, the time went from 09:26:46 to 09:43:26.
Here's another approach using Howard Hinnant's date library, which is on its way into C++2a.
#include <iostream>
#include "date/date.h"
using namespace std::chrono_literals;
// Time point representing the start of today:
auto today = date::floor<date::days>(std::chrono::system_clock::now());
auto someTp = today + 23h; // Today, at 23h00
auto anotherTp = someTp - 5min - 5s; // ... should be self-explanatory now :)
std::cout << date::format("%b-%d-%Y %T\n", anotherTp);
If you want to expose the manipulation of time points via a user interface, the compile-time constructs 23h, 5min and so on are of course not available. Those literals construct std::chrono::duration objects, so you need a mechanism to turn user input into equivalent instances.

c++ strptime ignores Timezone when parsing

When I run the following code strptime seems to ignore the timezone value. Just sets value of the local timezone (which is +10).
This is the output, (Running on Linux, compiled with gcc 4.6.3):
-----------2013-04-24T9:47:06+400 - %Y-%m-%dT%H:%M:%S%z
TM Break H:9 is DST:0 GMT Off:0
The epoch value: 1366760826
DateTime in String: 04/24/13 - 09:47AM +1000
-----------2013-04-24T11:47:06+800 - %Y-%m-%dT%H:%M:%S%z
TM Break H:11 is DST:0 GMT Off:36000
The epoch value: 1366768026
DateTime in String: 04/24/13 - 11:47AM +1000
-----------2013-04-24T9:47:06+0 - %Y-%m-%dT%H:%M:%S%z
TM Break H:9 is DST:0 GMT Off:36000
The epoch value: 1366760826
DateTime in String: 04/24/13 - 09:47AM +1000
-----------2013-04-24T9:47:06+4 - %Y-%m-%dT%H:%M:%S%z
TM Break H:9 is DST:0 GMT Off:36000
The epoch value: 1366760826
DateTime in String: 04/24/13 - 09:47AM +1000
This is the code:
void date_Test(){
string dateStrings[] = {"2013-04-24T9:47:06+400"
, "2013-04-24T11:47:06+800"
, "2013-04-24T9:47:06+0"
, "2013-04-24T9:47:06+4"};
string formatStrings[] = {"%Y-%m-%dT%H:%M:%S%z"
, "%Y-%m-%dT%H:%M:%S%z"
, "%Y-%m-%dT%H:%M:%S%z"
, "%Y-%m-%dT%H:%M:%S%z"};
process_Timezone(dateStrings, formatStrings);
}
void process_Timezone(string dateStrings[], string formatStrings[]){
int num = 4;
for (int i = 0; i < num; i++) {
cout << endl << "-----------" << dateStrings[i] << " - " << formatStrings[i] << endl;
tm *dtm = new tm;
strptime(dateStrings[i].c_str(), formatStrings[i].c_str(), dtm);
cout << "TM Break \tH:" << dtm->tm_hour << " is DST:" << dtm->tm_isdst << " GMT Off:" << dtm->tm_gmtoff << endl;
time_t ep_dt = mktime(dtm);
cout << "The epoch value: \t" << ep_dt << endl;
char buffer[40];
strftime(buffer, 40,"%x - %I:%M%p %z", dtm);
cout << "DateTime in String: \t" << buffer << endl;
delete dtm;
}
}
According to http://en.wikipedia.org/wiki/ISO_8601 your one and three digit timezone offsets are not valid ISO 8601 values (the form used by strptime at least on Linux), which requires hh[:][mm] as the format.
Might be late, but strptime does correctly parse the %z specifier, it stores it in tm.tm_gmtoff variable. This variable is not in the standard, but a gnu extension. mktime does not make use of this variable.
You can try
strptime(<time_string>, dtm);
std::cout << dtm->tm_gmtoff;
to see the parsed time offset.
Note .tm_gmtoff is in second, so to get the correct epoch time, you can do
auto offset = dtm->tm_gmtoff;
auto _epoch = mktime(dtm);
auto final_epoch = _epoch + offset
Sources
https://www.gnu.org/software/libc/manual/html_node/Broken_002ddown-Time.html
https://github.com/andikleen/glibc/blob/master/time/strptime_l.c#L749
The answer from #Nguyen Ha Kien helped me, but to get the right epoch time, I needed to factor in my local UTC offset too, after it's been filled in by mktime, and not override libc's determination of whether DST was active at the specified time here:
int offset = dtm->tm_gmtoff;
// Ask libc to work out whether the local timezone is in DST at this time.
dtm->tm_isdst = -1;
time_t ep_dt = mktime(dtm);
ep_dt -= offset - dtm->tm_gmtoff;
int rather than auto just because I'm compiling on a 2011 era setup, hopefully like that used by pt123 originally, to show that the answer from Mark B gets strptime to parse tm_gmtoff for me:
(ia32)martind#sirius:~/playpen$ ls /lib/libc-2.11.3.so
/lib/libc-2.11.3.so
(ia32)martind#sirius:~/playpen$ gcc -v
...
gcc version 4.4.5 (Debian 4.4.5-8)
(ia32)martind#sirius:~/playpen$ g++ c-strptime-ignores-timezone-when-parsing.cpp
(ia32)martind#sirius:~/playpen$ ./a.out
-----------2013-04-24T9:47:06+0400 - %Y-%m-%dT%H:%M:%S%z
TM Break H:9 is DST:0 GMT Off:14400
The only additional code was:
#include <iostream>
#include <string>
using namespace std;
void process_Timezone(string dateStrings[], string formatStrings[]);
void date_Test();
int main(){
date_Test();
}