Two days ago, I set out to code something that I thought would be very simple: format a date string as saved in a DATE field in mysql from "YYYY-MM-DD" to a user supplied format. Since then, I have been stuck in problems with locales.
#include <iostream>
#include <ctime>
std::string fdate(std::string date, std::string format) {
char buf[50];
int year, month, day;
std::tm tm;
sscanf(date.c_str(), "%d-%d-%d", &year, &month, &day);
tm.tm_year = year - 1900;
tm.tm_mon = month - 1;
tm.tm_mday = day;
// std::cout << "std::cout locale: " << std::cout.getloc().name() << std::endl;
strftime(buf, sizeof(buf), format.c_str(), &tm);
return buf;
}
int main() {
std::locale::global(std::locale(""));
std::cout.imbue(std::locale());
std::string format("%a %d %b %Y");
std::string day("2018-04-14");
std::string formatted_day = fdate(day, format);
std::cout << "std::cout locale: " << std::cout.getloc().name() << std::endl;
std::cout << day << " => " << formatted_day << std::endl;
return 0;
}
The above code outputs:
std::cout locale: en_GB.utf8
2018-04-14 => ? 14 Apr 2018
Notice that "%a" is supposed to output the short English name of the day of the week, namely "Sun". Instead a "?" is output, because for some reasons that I do not understand, the locale information needed by strftime() is missing.
If I uncomment the line commented out in the method above, the output becomes:
std::cout locale: en_GB.utf8
std::cout locale: en_GB.utf8
2018-04-14 => Sun 14 Apr 2018
Over the past two days, I have tried many different variations, but the code sample above highlights one occurrence that puzzles me a lot.
Why adding a std::cout statement within the method "fixes" the problem?
What better method is there to achieve what I originally set out to do?
Edit: actually 2018-04-14 is a Saturday! So, I am making another wrong assumption in my code. How to fill in the missing field of std::tm? Isn't there anything in the std library capable of computing the day of the week given a YYYY-MM-DD date?
Related
I want to get the date, month and year information from the string.
Example Date String: Thu, 30 Jul 2020 00:51:08 -0700 (PDT)
PDT here is for Pacific Daylight time. This string offset (-0700) can change based on system timezone when the file was created.
I need to write a c++ program to extract date, month and year from this string.
Any thoughts on how to go about this?
This is a story of evolution. The correct answer greatly depends on your current toolset (how modern it is). And even if it is completely modern, there are still better tools coming.
Homo habilis
In C++98 we could stand upright. And we had tools to scan ints out of arrays of chars. scanf was the tool to do this. This result was not type safe, but we could scan ints and strings and then reinterpret those values as the components of a date: year, month and day. This might look something like this:
#include <cstdio>
#include <cstring>
#include <iostream>
int
main()
{
using namespace std;
string s = "Thu, 30 Jul 2020 00:51:08 -0700 (PDT)";
char const* months[] = {"Jan", "Feb", "Mar", "Apr", "May", "Jun",
"Jul", "Aug", "Sep", "Oct", "Nov", "Dec"};
char wd[4] = {};
int d;
char mon[4] = {};
int y;
sscanf(s.c_str(), "%s %d %s %d", wd, &d, mon, &y);
int m;
for (m = 0; m < 12; ++m)
if (strcmp(months[m], mon) == 0)
break;
++m;
cout << y << '\n';
cout << m << '\n';
cout << d << '\n';
}
This outputs:
2020
7
30
Notes:
The " 00:51:08 -0700 (PDT)" is never even parsed. It could be parsed. But it is a lot more work.
There's no error checking. This might be a valid date or might not.
There's no type safety. The results are just ints and if you mix them up, it's a run-time error, not a compile-time error.
Neanderthal
Using C++98, there's also a popular but non-standard solution: strptime.
#include <time.h>
#include <iostream>
int
main()
{
using namespace std;
string s = "Thu, 30 Jul 2020 00:51:08 -0700 (PDT)";
tm tm;
strptime(s.c_str(), "%a, %d %b %Y %T", &tm);
cout << tm.tm_year + 1900 << '\n';
cout << tm.tm_mon + 1 << '\n';
cout << tm.tm_mday << '\n';
cout << tm.tm_hour << '\n';
cout << tm.tm_min << '\n';
cout << tm.tm_sec << '\n';
}
strptime is in the POSIX standard, but not in the C or C++ standards. It is also supported by MS Visual Studio. So it is a popular extension. And with good reason. It is much higher level, and puts the results into a struct tm: A type representing a date/time; the beginnings of type safety.
Output:
2020
7
30
0
51
8
There are still some problems:
" -0700 (PDT)" is never parsed. There's no way to ask strptime to do this.
There are weird and inconsistent offsets on the different fields of tm. For example the month is zero-based and the day is one-based. But at least it knows how to parse the time too, and relatively easily.
Error checking is there but easy to ignore. strptime returns NULL if something bad happens.
Cro-Magnon
With C++11 arrived an actual C++ wrapper around strptime that was officially recognized by the C++ standard with std::get_time:
#include <iomanip>
#include <iostream>
#include <sstream>
int
main()
{
using namespace std;
string s = "Thu, 30 Jul 2020 00:51:08 -0700 (PDT)";
istringstream in{s};
in.exceptions(ios::failbit);
tm tm;
in >> get_time(&tm, "%a, %d %b %Y %T");
cout << tm.tm_year + 1900 << '\n';
cout << tm.tm_mon + 1 << '\n';
cout << tm.tm_mday << '\n';
cout << tm.tm_hour << '\n';
cout << tm.tm_min << '\n';
cout << tm.tm_sec << '\n';
}
With a C++ wrapper you can parse from streams, which gives you access to throwing an exception on parse failure. But it is still a simple wrapper and so the result is just a tm. This has the same weirdness as the previous solution.
The output is the same as in the previous solution:
2020
7
30
0
51
8
Homo sapiens
Though the strongly typed std::chrono time_point / duration system was introduced in C++11, it is not until C++20 that it is integrated with the civil calendar, gaining get_time-like functionality, and going far beyond that.
#include <chrono>
#include <iostream>
#include <sstream>
int
main()
{
using namespace std;
using namespace std::chrono;
string s = "Thu, 30 Jul 2020 00:51:08 -0700 (PDT)";
istringstream in{s};
in.exceptions(ios::failbit);
local_seconds t;
in >> parse("%a, %d %b %Y %T %z (%Z)", t);
auto td = floor<days>(t);
year_month_day ymd{td};
hh_mm_ss hms{t-td};
cout << ymd << ' ' << hms << '\n';
cout << ymd.year() << '\n';
cout << ymd.month() << '\n';
cout << ymd.day() << '\n';
cout << hms.hours() << '\n';
cout << hms.minutes() << '\n';
cout << hms.seconds() << '\n';
}
Output:
2020-07-30 00:51:08
2020
Jul
30
0h
51min
8s
The first thing to notice is the much stronger type-safety. No longer is there a need to convert everything to ints to print it out. And no longer is it necessary to convert to ints to do other operations such as arithmetic and comparison.
For example ymd.year() has type std::chrono::year, not int. If necessary, one can explicitly convert between these two representations. But it is generally unnecessary, and akin to a risky reinterpret_cast.
There are no longer unintuitive biases such as 1900, or zero-based counts in unexpected places.
Output generally includes the units for easier debugging.
The " -0700 (PDT)" is parsed here! These values are not used in the results, but they must be there, else there is a parse error. And if you want to get these values, they are available with very simple changes:
string abbrev;
minutes offset;
in >> parse("%a, %d %b %Y %T %z (%Z)", t, abbrev, offset);
...
cout << offset << '\n';
cout << abbrev << '\n';
Now the output includes:
-420min
PDT
If you need the fields in UTC, instead of in local time, that is one simple change:
sys_seconds t;
instead of:
local_seconds t;
Now the offset is subtracted from the parsed time point to result in a UTC time_point (a std::chrono::time_point based on system_clock) instead and the output changes to:
2020-07-30 07:51:08
2020
Jul
30
7h
51min
8s
This allows you to easily parse local times plus offset directly into system_clock::time_point.
Though not shipping yet (as I write this), vendors are working on implementing this. And in the meantime you can get this functionality with a free, open-source, header-only C++20 <chrono> preview library that works with C++11/14/17. Just add #include "date/date.h" and using namespace date; and everything just works. Though with C++11/14 you will need to substitute hh_mm_ss<seconds> hms{t-td}; for hh_mm_ss hms{t-td}; (lack of CTAD).
#include <time.h>
char *strptime(const char *buf, const char *format, struct tm *tm);
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;
}
} // ()
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();
}
I want to convert a string in the format of "20160907-05:00:54.123" into milliseconds.
I know that strptime is not available in Windows and I want to run my program in both windows and linux. I can't use third party libraries as well.
I can tokenize the string and convert it. But is there a more elegant way like using the strptime to do so?
What about std::sscanf?
#include <iostream>
#include <cstring>
int main() {
const char *str_time = "20160907-05:00:54.123";
unsigned int year, month, day, hour, minute, second, miliseconds;
if (std::sscanf(str_time, "%4u%2u%2u-%2u:%2u:%2u.%3u", &year, &month,
&day, &hour, &minute, &second,&miliseconds) != 7)
{
std::cout << "Parse failed" << std::endl;
}
else
{
std::cout << year << month << day << "-" << hour << ":"
<< minute << ":" << second << "." << miliseconds
<< std::endl;
}
}
Output (ideone):
201697-5:0:54.123.
However, you should make sure the input is valid (for example, day can be in the range of [0,99]).
Too bad about no 3rd party libraries, because here is one (MIT license) that is just a single header, runs on linux and Windows, and handles the milliseconds seamlessly:
#include "date.h"
#include <iostream>
#include <sstream>
int
main()
{
date::sys_time<std::chrono::milliseconds> tp;
std::istringstream in{"20160907-05:00:54.123"};
date::parse(in, "%Y%m%d-%T", tp);
std::cout << tp.time_since_epoch().count() << '\n';
}
This outputs:
1473224454123
Error checking is done for you. The stream will fail() if the date is invalid.
date::sys_time<std::chrono::milliseconds> is a type alias for std::chrono::time_point<std::chrono::system_clock, std::chrono::milliseconds>. I.e. it is from the family of system_clock::time_point, just milliseconds precision.
Fully documented:
https://howardhinnant.github.io/date/date.html
Doesn't get much more elegant than this.
Given the format of your string, it is fairly easy to parse it as follows (although a regex or get_time might be more elegant):
tm t;
t.tm_year = stoi(s.substr(0, 4));
t.tm_mon = stoi(s.substr(4, 2));
t.tm_mday = stoi(s.substr(6, 2));
t.tm_hour = stoi(s.substr(9, 2));
t.tm_min = stoi(s.substr(12, 2));
t.tm_sec = 0;
double sec = stod(s.substr(15));
Finding the time since the epoch can be done with mktime:
mktime(&t) + sec * 1000
Note that the fractional seconds need to be handled differently - unfortunately, tm has only integer seconds.
(See the full code here.)
Edit
As Mine and Panagiotis Kanavos correctly note in the comments, Visual C++ apparently supports get_time for quite a while, and it's much shorter with it (note that the fractional seconds need to be handled the same way, though).
I am trying to save std::time_point into a std::stream and read it back. One problem is that using the standard functions 'loses' an hour somewhere. I.e., the time I read is 1 hour behind the time I write. I suspect that I need to set up daylight saving somewhere. I put together a small program that prints time to a std::stringstream and reads it back.
#include <iomanip>
#include <iostream>
#include <sstream>
#include <chrono>
#include <ctime>
using std::chrono::system_clock;
namespace chrono = std::chrono;
void test();
int main(int argc, char** argv)
{
std::stringstream ss;
auto start = system_clock::now();
std::time_t ts = system_clock::to_time_t(start);
std::tm time_out = *std::localtime(&ts);
ss << std::put_time(&time_out, "%Y-%m-%d %H:%M:%S %Z") << '\n';
std::cout << ss.str() << std::endl;
std::tm time_in;
ss >> std::get_time(&time_in, "%Y-%m-%d %H:%M:%S %Z");
std::cout << "Are time dsts equal? : " <<
(time_out.tm_isdst == time_in.tm_isdst) << '\n';
std::time_t rawTime = std::mktime(&time_in);
auto end = std::chrono::system_clock::from_time_t(rawTime);
std::cout << "Are time points equal? : " << (start == end) << '\n';
// print the trouble makers
std::time_t start_time = system_clock::to_time_t(start);
std::time_t end_time = system_clock::to_time_t(end);
std::cout << "times: \n"
<< '\t' << std::put_time(std::localtime(&start_time), "%c %z") << '\n'
<< '\t' << std::put_time(std::localtime(&end_time), "%c %z") << '\n';
// this is a source of strange behaviour...
// std::cout << "Difference: "
// << chrono::duration_cast<chrono::seconds>(start - end).count()
// << std::endl;
return 0;
}
The strangest thing is that the program prints the following:
Are time dsts equal? : 1
Are time points equal? : 0
times:
Tue Dec 11 19:26:24 2012 +0000
Tue Dec 11 19:26:24 2012 +0000
And when I uncomment the 3 lines at the end of the program (printing the difference between the time points) the result is:
Are time dsts equal? : 0
Are time points equal? : 0
times:
Tue Dec 11 19:29:40 2012 +0000
Tue Dec 11 18:29:40 2012 +0000
Difference: 3600
Notice that dst(daylight saving time) is suddenly not equal and neither are the times.
I am using libc++ on Mac OS X 10.8.2 with XCode46-DP2. The clang++ versions I am using are Apple clang version 4.1 and clang version 3.2 (trunk 167239)
I guess my questions are:
A) As to the 1 hour difference, is this a bug in my library or am I not using the standard functions correctly? (The latter would not surprise me...)
B) What is going on with the code when I uncomment the three lines at the end of my program? This looks like a bug to me. Anyone care to try it on their platform?
I think we're looking at a bug with the %Z specifier, and maybe with the %z specifier as well, not sure yet.
I will look into the cause of these bugs more. However I wanted to go ahead and post to get you a workaround. I believe if you zero-initialize your input tm, and always assume it is with respect to your local timezone, then you will eliminate your errors:
std::tm time_in{0};