I am getting incorrect results using get_time to parse a date with a PM modifier.
To be sure that I am using the correct modifier (even though it should not depend on the locale at all), I generate the time using put_time in this example code:
#include <iostream>
#include <sstream>
#include <iomanip>
using namespace std;
int main() {
// Initialize time from 24-format
tm tm;
strptime("16:33:45", "%H:%M:%S", &tm);
// Define 12h time format
string fmt("%I:%M:%S%p");
// Convert time to 12h format and print - looks good!
ostringstream oss;
oss << put_time(&tm, fmt.c_str());
cout << oss.str() << endl;
// Read time in 12h format
istringstream iss(oss.str());
iss >> get_time(&tm, fmt.c_str());
// Result is wrong!
cout << iss.fail() << " (Fail)" << endl;
cout << tm.tm_hour << endl;
// Read time in 12h format using strptime
strptime(iss.str().c_str(), fmt.c_str(), &tm);
// Result is correct!
cout << "Correct" << endl;
cout << tm.tm_hour << endl;
}
Output:
04:33:45PM
1 (Fail)
4
Correct
16
See also https://coliru.stacked-crooked.com/view?id=c4f75f14fdf53095
I have come across an (unconfirmed) GCC bug from 2018 which reads very similarly, namely, https://gcc.gnu.org/bugzilla/show_bug.cgi?id=84216. Also, I have found is this a bug in std::get_time? which according to the only answer is related to some Microsoft implementation I am not aware of using.
strptime is able to parse the time correctly.
Is this a bug somewhere, or am I using get_time incorrectly?
Related
How do I get a uint unix timestamp in C++? I've googled a bit and it seems that most methods are looking for more convoluted ways to represent time. Can't I just get it as a uint?
C++20 introduced a guarantee that time_since_epoch is relative to the UNIX epoch, and cppreference.com gives an example that I've distilled to the relevant code, and changed to units of seconds rather than hours:
#include <iostream>
#include <chrono>
int main()
{
const auto p1 = std::chrono::system_clock::now();
std::cout << "seconds since epoch: "
<< std::chrono::duration_cast<std::chrono::seconds>(
p1.time_since_epoch()).count() << '\n';
}
Using C++17 or earlier, time() is the simplest function - seconds since Epoch, which for Linux and UNIX at least would be the UNIX epoch. Linux manpage here.
The cppreference page linked above gives this example:
#include <ctime>
#include <iostream>
int main()
{
std::time_t result = std::time(nullptr);
std::cout << std::asctime(std::localtime(&result))
<< result << " seconds since the Epoch\n";
}
#include<iostream>
#include<ctime>
int main()
{
std::time_t t = std::time(0); // t is an integer type
std::cout << t << " seconds since 01-Jan-1970\n";
return 0;
}
The most common advice is wrong, you can't just rely on time(). That's used for relative timing: ISO C++ doesn't specify that 1970-01-01T00:00Z is time_t(0)
What's worse is that you can't easily figure it out, either. Sure, you can find the calendar date of time_t(0) with gmtime, but what are you going to do if that's 2000-01-01T00:00Z ? How many seconds were there between 1970-01-01T00:00Z and 2000-01-01T00:00Z? It's certainly no multiple of 60, due to leap seconds.
As this is the first result on google and there's no C++20 answer yet, here's how to use std::chrono to do this:
#include <chrono>
//...
using namespace std::chrono;
int64_t timestamp = duration_cast<milliseconds>(system_clock::now().time_since_epoch()).count();
In versions of C++ before 20, system_clock's epoch being Unix epoch is a de-facto convention, but it's not standardized. If you're not on C++20, use at your own risk.
#include <iostream>
#include <sys/time.h>
using namespace std;
int main ()
{
unsigned long int sec= time(NULL);
cout<<sec<<endl;
}
I created a global define with more information:
#include <iostream>
#include <ctime>
#include <iomanip>
#define __FILENAME__ (__builtin_strrchr(__FILE__, '/') ? __builtin_strrchr(__FILE__, '/') + 1 : __FILE__) // only show filename and not it's path (less clutter)
#define INFO std::cout << std::put_time(std::localtime(&time_now), "%y-%m-%d %OH:%OM:%OS") << " [INFO] " << __FILENAME__ << "(" << __FUNCTION__ << ":" << __LINE__ << ") >> "
#define ERROR std::cout << std::put_time(std::localtime(&time_now), "%y-%m-%d %OH:%OM:%OS") << " [ERROR] " << __FILENAME__ << "(" << __FUNCTION__ << ":" << __LINE__ << ") >> "
static std::time_t time_now = std::time(nullptr);
Use it like this:
INFO << "Hello world" << std::endl;
ERROR << "Goodbye world" << std::endl;
Sample output:
16-06-23 21:33:19 [INFO] main.cpp(main:6) >> Hello world
16-06-23 21:33:19 [ERROR] main.cpp(main:7) >> Goodbye world
Put these lines in your header file. I find this very useful for debugging, etc.
Windows uses a different epoch and time units: see
Convert Windows Filetime to second in Unix/Linux
What std::time() returns on Windows is (as yet) unknown to me (;-))
Hello I have the following date that I am consuming from an api
string sToParse = "2020-04-17T09:30:00-04:00";
which should be in human form "Friday, April 17, 2020 08:30:00" central time
or epoch of 1587130200
however this code
cout << "raw: " << sToParse << endl;
static const std::string dateTimeFormat { "%Y-%m-%dT%H:%M:%S%Z" };
istringstream ss{ sToParse };
tm dt;
ss >> get_time(&dt, dateTimeFormat.c_str());
cout << mktime(&dt) << endl;
Gives me an epoch of 1587137400 which is a human format of "Friday, April 17, 2020 10:30:00 AM" which is two hours difference. How do i get the %Z to process the timezone appropriately?
Thanks in advance for any help you can give
C++20 will do this:
#include <chrono>
#include <iostream>
#include <sstream>
#include <string>
int
main()
{
using namespace std;
using namespace std::chrono;
string sToParse = "2020-04-17T09:30:00-04:00";
cout << "raw: " << sToParse << endl;
static const std::string dateTimeFormat { "%Y-%m-%dT%H:%M:%S%z" };
istringstream ss{ sToParse };
sys_seconds dt;
ss >> parse(dateTimeFormat, dt);
cout << dt << '\n';
cout << dt.time_since_epoch() << '\n';
}
Output:
2020-04-17 13:30:00
1587130200s
This gets the "epoch time" you're looking for. The big difference is the use of %z instead of %Z. %z is the command for parsing the offset. %Z parses time zone abbreviation.
This doesn't get the "human time" you're expecting. The time printed out above is UTC. This is clearly correct on inspection: 13:30:00 is 4 hours after 09:30:00.
If you are wanting local time, it would be 2020-04-17 09:30:00, same as the input. To come up with 08:30:00 would require more information than you are providing above (e.g. output in some time zone other than that which has a UTC offset of -4h at this local time).
Also it is in general not possible to go from a UTC offset to a time zone name or abbreviation because more than one time zone will generally have the same UTC offset at any point in time.
If the C++20 <chrono> header isn't available for you (to the best of my knowledge it is not yet available anywhere), you can use a free, open-source preview of C+++20 <chrono>. For this problem, one only needs the header-only "date.h" from this preview. And everything is in namespace date instead of namespace std::chrono. It would look like this:
#include "date/date.h"
#include <chrono>
#include <iostream>
#include <sstream>
#include <string>
int
main()
{
using namespace date;
using namespace std;
using namespace std::chrono;
string sToParse = "2020-04-17T09:30:00-04:00";
cout << "raw: " << sToParse << endl;
static const std::string dateTimeFormat { "%Y-%m-%dT%H:%M:%S%z" };
istringstream ss{ sToParse };
sys_seconds dt;
ss >> parse(dateTimeFormat, dt);
cout << dt << '\n';
cout << dt.time_since_epoch() << '\n';
}
Update
With the new knowledge that the "expected human form" of the time should be US Central Time ("America/Chicago" in IANA terms), I'm updating the example code to show how that can be handled.
#include "date/tz.h"
#include <chrono>
#include <iostream>
#include <sstream>
#include <string>
int
main()
{
using namespace date;
using namespace std;
using namespace std::chrono;
string sToParse = "2020-04-17T09:30:00-04:00";
cout << "raw: " << sToParse << endl;
static const std::string dateTimeFormat { "%FT%T%z" };
istringstream ss{ sToParse };
sys_seconds utc;
ss >> parse(dateTimeFormat, utc);
zoned_seconds cst{"America/Chicago", utc};
cout << utc.time_since_epoch() << '\n';
cout << format("%F %T %Z\n", utc);
cout << format("%F %T %Z\n", cst);
}
A few changes above:
A new header is required to handle the time zone issues: "tz.h" (only in the C++20 preview library, not in C++20).
For parsing I've substituted "%Y-%m-%dT%H:%M:%S%z" for "%FT%T%z". These are equivalent. %F is just shorthand for %Y-%m-%d and %T is shorthand for %H:%M:%S. This change is not required.
I've renamed dt to utc to emphasize that this variable holds a UTC time point. This change is not required.
The new line constructs a zoned_time (with seconds precision) with the desired destination time zone ("America/Chicago"), and the UTC time point. This creates an object that knows all about the local time at this time point and in this time zone.
Then everything is printed out using the date::format function and the formatting string "%F %T %Z". %Z is used to output the time zone abbreviation to make the output more readable. In C++20, this will be std::format, and the formatting string will be "{:%F %T %Z}".
The output is now:
raw: 2020-04-17T09:30:00-04:00
1587130200s
2020-04-17 13:30:00 UTC
2020-04-17 08:30:00 CDT
If your computer's current time zone setting happens to be US Central, then the line that constructs zoned_seconds can also look like:
zoned_seconds cst{current_zone(), utc};
Or conversely, you can use this line to find the local time at utc in any time zone which your computer is set to.
Note that whether the time zone is specified with a name such as "America/Chicago", or with current_zone(), any changes of UTC offset within the specified time zone (such as daylight saving time) will be correctly taken into account.
With the preview C+++20 library, the use of the header "tz.h" is not header-only. It requires a single source file, tz.cpp. Here are instructions on how to compile it. But if you are using a fully conforming C++20 <chrono>, then the above will just work by removing #include "date/tz.h", using namespace date;, and adjusting the formatting string as noted in the 5th bullet above.
What's a standard way to get a date time in ISO8601 format on Windows using C++? Specifically, I would like it to be formatted as:
2017-02-22T10:00:00.123-05:00
2017-02-22T10:00:00.123 >>> -05:00 <<< # how to print the offset?
I was looking into combining the output of GetLocalTime and GetTimeZoneInformation, but this looks esoteric. There are similar questions on SO, however, I've not found a single one that prints UTC offset in a desired format. Is there a better approach?
The format specifier %z gives you the timezone offset as described in the documentation (e.g. MSDN on strftime) but lefts out the ':'. You can use it like this to get the ':' into your string:
struct tm tmNow;
time_t now = time(NULL); // Get the current time
_localtime64_s(&tmNow, &now);
char bufferTime[26];
char bufferTimezoneOffset[6];
size_t tsizTime = strftime(bufferTime, 26, "%Y-%m-%dT%H:%M:%S", &tmNow); // The current time formatted "2017-02-22T10:00:00"
size_t tsizOffset = strftime(bufferTimezoneOffset, 6, "%z", &tmNow); // The timezone offset -0500
strncpy_s(&bufferTime[tsizTime], 26, bufferTimezoneOffset, 3); // Adds the hour part of the timezone offset
bufferTime[tsizTime + 3] = ':'; // insert ':'
strncpy_s(&bufferTime[tsizTime + 4], 26, &bufferTimezoneOffset[3], 3); // Adds the minutes part of the timezone offset
puts(bufferTime); // Your output: "2017-02-22T10:00:00-05:00"
I left out the milliseconds, as they are not part of the localtime as far as I know.
Maybe something like this. We call GetLocalTime and GetTimeZoneInformation then pass it to the function which returns formatted string.
This is written quickly, not tested besides observing the fact it returns correct result on my machine now. It operates on the fact that SYSTEMTIME has a member Bias where UTC = Localtime + Bias and Bias is set in minutes. So get hours by dividing by 60 and taking absolute value of that. Then we get the minutes in similar way and set the sign depending on if Bias > 0
#include <Windows.h>
#include <string>
#include <sstream>
#include <iomanip>
#include <cmath>
std::string format_system_time(const SYSTEMTIME& sys_time, const TIME_ZONE_INFORMATION& time_zone)
{
std::ostringstream formatted_date_time;
formatted_date_time << std::setfill('0');
formatted_date_time << sys_time.wYear << "-" << std::setw(2) << sys_time.wMonth << "-" <<
std::setw(2) << sys_time.wDay << "T" << std::setw(2) << sys_time.wHour << ":" <<
std::setw(2) << sys_time.wMinute << ":" << std::setw(2) << sys_time.wSecond << "." <<
std::setw(3) << sys_time.wMilliseconds;
//UTC = localtime + bias; bias is in minutes
int utc_offset_hours = time_zone.Bias / 60;
int utc_offset_minutes = std::abs(time_zone.Bias - (utc_offset_hours * 60));
char offset_sign = time_zone.Bias > 0 ? '-' : '+';
formatted_date_time << offset_sign << std::setw(2) << std::abs(utc_offset_hours) << ":" << utc_offset_minutes;
return formatted_date_time.str();
}
int main(int argc, char* argv[])
{
SYSTEMTIME date_and_time;
GetLocalTime(&date_and_time);
TIME_ZONE_INFORMATION time_zone;
GetTimeZoneInformation(&time_zone);
auto& formatted_date_time = format_system_time(date_and_time, time_zone);
return 0;
}
I don't think there is a drop-in solution for c++ on Windows. The closest you can get is InternetTimeFromSystemTime but it is only documented to support RFC1123.
You probably have to code it yourself with GetLocalTime + GetTimeZoneInformation + wsprintf (or GetTimeZoneInformationForYear if you are not dealing with the current time).
Using Howard Hinnant's free, open-source timezone library, which works on VS-2013 and later, but does require some installation:
#include "tz.h"
#include <iostream>
int
main()
{
using namespace std;
using namespace std::chrono;
using namespace date;
auto zt = make_zoned(current_zone(), floor<milliseconds>(system_clock::now()));
cout << format("%FT%T%Ez\n", zt);
}
This just output for me:
2017-02-22T17:29:03.859-05:00
Is there a simpler way to do a time stamp for saving a file/creating a directory as a date time stamp ?
only using standard library (not boost). Is there a faster way to do it ?
This is my current code
std::chrono::system_clock::time_point now = std::chrono::system_clock::now();
std::time_t tt = std::chrono::system_clock::to_time_t(now);
tm utc_tm = *gmtime(&tt);
oname.str("");
oname << (utc_tm.tm_year + 1900) << '-' << std::setfill('0') << std::setw(2) << (utc_tm.tm_mon + 1) << '-' << std::setfill('0') << std::setw(2) << utc_tm.tm_mday << " " << std::setfill('0') << std::setw(2)<< utc_tm.tm_hour <<':' << std::setfill('0') << std::setw(2) << utc_tm.tm_min <<':' << std::setfill('0') << std::setw(2) << utc_tm.tm_sec;
ts = oname.str();
There is a less tortuous way:
#include <string>
#include <ctime>
std::string get_timestamp()
{
auto now = std::time(nullptr);
char buf[sizeof("YYYY-MM-DD HH:MM:SS")];
return std::string(buf,buf +
std::strftime(buf,sizeof(buf),"%F %T",std::gmtime(&now)));
}
It is very probably also faster, because it is less tortuous, but that is
also very probably immaterial in a setting where disc I/O is in play.
This gives you the same timestamps as your own code, e.g.
2015-03-28 10:48:45
See std::time and
std::strftime to
understand how the desired formatting is achieved and note that std::strftime
returns the length of the string it has composed, excluding its nul-terminator.
This code is standard, but if you are working with MS VC++ 2013 or later then
you could also consider the use of std::put_time,
as in:
#include <iomanip>
#include <sstream>
#include <string>
#include <ctime>
std::string get_timestamp()
{
auto now = std::time(nullptr);
std::ostringstream os;
os << std::put_time(std::gmtime(&now),"%F %T");
return os.str();
}
which is simpler still. (I have not tested that.) std::put_time however
is unsupported by gcc as of 4.9.
Seemingly you want your timestamps formatted as YYYY-MM-DD HH:MM:SS. If they
are to be used in filenames, it would be more prudent to keep them free of spaces:
perhaps YYYY-MM-DD_HH:MM:SS.
I'm having trouble converting a ptime object from boost into a string to be passed in to a function. I have found multiple similar other threads in regards to outputing a boost time object to a string (mostly to cout) but none of what I've found on them are working.
It appears the easiest way is inserting the ptime object into a stringstream and then using the stringstream's string. I have also attempted to imbue the stringstream with a time_facet, as some of the answers on other threads suggest. However, I am unable to create a time_facet object. It gives me the error that the argument list for the class template is missing. What is confusing is the nowhere on the internet have I found any mention of an argument list for time_facet, and even boost's documentation page shows that the default constructor for a time_facet is merely time_facet().
Below is a simple version of what I have tried:
#include <boost/date_time/posix_time/posix_time.hpp>
#include <boost/date_time/posix_time/posix_time_io.hpp>
boost::posix_time::ptime time = boost::posix_time::time_from_string("1981-08-20 08:05:00");
std::stringstream sstream;
sstream << time;
_updateStatement->setString(1, (sql::SQLString)sstream.str());
The insertion of time into the stringstream gives me a bunch of compilation errors in the vein of
error C2220: warning treated as error - no 'object' file generated C:\code\trunk\Development\External\boost\include\boost/date_time/time_facet.hpp(247) :while compiling class template member function 'boost::date_time::time_facet<time_type,CharT>::time_facet(size_t)'
with
[
time_type=boost::posix_time::ptime,
CharT=char
]
despite the fact that I haven't used any time_facet objects.
When I DO try to do this with a time_facet object, I add in
sstream.imbue(std::locale(sstream.getloc(), new boost::date_time::time_facet("%Y-%m-%d %H:%M:%S")));
before inserting the time into the stringstream. The errors for that are that it wants an argument list as mentioned at the top of this post.
Is there perhaps a function in boost that is the reverse of boost::posix_time::time_from_string()? If not, any other help would be appreciated. Thank you.
The Boost.Date_Time library provides the following ptime to std::string conversions within the boost::posix_time namespace:
std::string to_simple_string(ptime) returns a string in the form of YYYY-mmm-DD HH:MM:SS.fffffffff format where mmm is the three character month name.
std::string to_iso_string(ptime) returns a string in the form of YYYYMMDDTHHMMSS,fffffffff where T is the date-time separator.
std::string to_iso_extended_string(ptime) returns a string in the form of YYYY-MM-DDTHH:MM:SS,fffffffff where T is the date-time separator.
Additionally, stream insertion and extraction operators are provided, allowing ptime to be inserted or extracted from a stream. The input and output formats can be customized by constructing facets with various format flags, and then imbuing the stream with the facet.
Based on the compile error (C2220), the compiler is set to treat all warnings as errors. In some cases, the Boost libraries will compile with warnings. Consider assessing the severity of the actual warning, and handling it appropriately from there. For example, if the warning is trivial, it may be acceptable to use a warning pragma to disable or suppress the specific warning.
Here is a complete example demonstrating converting ptime to a string via its provided conversion functions and stream operators.
#include <iostream>
#include <locale>
#include <string>
#include <boost/date_time/posix_time/posix_time.hpp>
#include <boost/date_time/posix_time/posix_time_io.hpp>
int main()
{
const boost::posix_time::ptime time =
boost::posix_time::time_from_string("1981-08-20 08:05:00");
// ptime to string.
const std::string str_time = to_simple_string(time);
std::cout << str_time << std::endl;
// ptime to stringstream to string.
std::stringstream stream;
stream << time;
std::cout << stream.str() << std::endl;
stream.str("");
// Use a facet to display time in a custom format (only hour and minutes).
boost::posix_time::time_facet* facet = new boost::posix_time::time_facet();
facet->format("%H:%M");
stream.imbue(std::locale(std::locale::classic(), facet));
stream << time;
std::cout << stream.str() << std::endl;
}
Which produces the following output:
1981-Aug-20 08:05:00
1981-Aug-20 08:05:00
08:05
My usage using release 1.55
#include <iostream>
#include <boost/date_time/gregorian/gregorian.hpp>
#include <boost/date_time.hpp>
#include <boost/date_time/posix_time/posix_time.hpp>
int main()
{
boost::gregorian::date dayte(boost::gregorian::day_clock::local_day());
boost::posix_time::ptime midnight(dayte);
boost::posix_time::ptime
now(boost::posix_time::microsec_clock::local_time());
boost::posix_time::time_duration td = now - midnight;
std::stringstream sstream;
std::cout << dayte << std::endl;
std::cout << dayte.year() << "/" << dayte.month().as_number()
<< "/" << dayte.day() << std::endl;
std::cout << now << std::endl;
std::cout << td << std::endl;
std::cout << td.hours() << "/" << td.minutes() << "/"
<< td.seconds() << "/" << td.fractional_seconds() << std::endl;
sstream << dayte << std::endl;
sstream << dayte.year() << "/" << dayte.month().as_number()
<< "/" << dayte.day() << std::endl;
sstream << now << std::endl;
sstream << td << std::endl;
sstream << td.hours() << "/" << td.minutes() << "/" << td.seconds()
<< "/" << td.fractional_seconds() << std::endl;
std::cout << sstream.str();
}
Results:
2015-Oct-27
2015/10/27
2015-Oct-27 14:25:18.614684
14:25:18.614684
14/25/18/614684
2015-Oct-27
2015/10/27
2015-Oct-27 14:25:18.614684
14:25:18.614684