boost last_write_time is back one hour

Question

I have trying to solve a bug on the following code, where I get an hour difference from boost last_write_time.

To explain it better: I create a file, and then I try to extract the time it was created with boost::filesystem::path.

void PrintTime(boost::filesystem::path _file) {
    time_t sys_time{ last_write_time(_file) };
    ptime p_time{ boost::posix_time::from_time_t(sys_time) };
    boost::posix_time::time_duration time_dur{ p_time.time_of_day() };

    long h{ time_dur.hours() }; //1a
    long m{ time_dur.minutes() };
    long s{ time_dur.seconds() };

    //...print h, m, s.
    }

    //1a: Here when for example the time I expect is 12:34:56,
    //I always get 11:34:56

Any idea why is that? Is there timezone somewhere in boost last_write_time? My os displays the right time when I check the file through the system.

Answer 1

You have to translate to the "presentation" time-zone, like "when [you] check the file through the system". The timestamp from the filesystem is UTC time.

E.g. if you do

std::cout << boost::posix_time::second_clock::local_time() << "\n";
std::cout << boost::posix_time::second_clock::universal_time() << "\n";

you'll probably get

2018-Feb-27 16:03:12
2018-Feb-27 15:03:12

Fix:

#include <boost/date_time/c_local_time_adjustor.hpp>

void PrintTime(boost::filesystem::path _file) {
    using boost::posix_time::ptime;
    using adj = boost::date_time::c_local_adjustor<ptime>;

    time_t const sys_time = last_write_time(_file);
    ptime const utc       = boost::posix_time::from_time_t(sys_time);
    ptime const local     = adj::utc_to_local(utc);

DEMO

See it Live On Coliru

#include <boost/filesystem.hpp>
#include <boost/date_time/posix_time/posix_time_io.hpp>
#include <boost/date_time/posix_time/conversion.hpp>
#include <boost/date_time/posix_time/posix_time.hpp>
#include <boost/date_time/c_local_time_adjustor.hpp>

void PrintTime(boost::filesystem::path _file) {
    using boost::posix_time::ptime;
    using adj = boost::date_time::c_local_adjustor<ptime>;

    time_t const sys_time = last_write_time(_file);
    ptime const utc       = boost::posix_time::from_time_t(sys_time);
    ptime const local     = adj::utc_to_local(utc);

    std::cout << "utc: " << utc << "\n";
    std::cout << "local: " << local << "\n";
    {
        long h{ local.time_of_day().hours() };
        long m{ local.time_of_day().minutes() };
        long s{ local.time_of_day().seconds() };

        //...print h, m, s.
        std::cout << h << ":" << m << ":" << s << '\n';
    }
}

int main() {
    PrintTime("main.cpp");
}

Prints (on my system):

utc: 2018-Feb-27 15:19:45
local: 2018-Feb-27 16:19:45
16:19:45