How to convert std::chrono::time_point to calendar datetime string with fractional seconds?

Question

How to convert std::chrono::time_point to calendar datetime string with fractional seconds?

For example:

\"10-10-2012 12:38:40.123456\"         


        

Answer 1

Self-explanatory code follows which first creates a std::tm corresponding to 10-10-2012 12:38:40, converts that to a std::chrono::system_clock::time_point, adds 0.123456 seconds, and then prints that out by converting back to a std::tm. How to handle the fractional seconds is in the very last step.

#include 
#include 
#include 

int main()
{
    // Create 10-10-2012 12:38:40 UTC as a std::tm
    std::tm tm = {0};
    tm.tm_sec = 40;
    tm.tm_min = 38;
    tm.tm_hour = 12;
    tm.tm_mday = 10;
    tm.tm_mon = 9;
    tm.tm_year = 112;
    tm.tm_isdst = -1;
    // Convert std::tm to std::time_t (popular extension)
    std::time_t tt = timegm(&tm);
    // Convert std::time_t to std::chrono::system_clock::time_point
    std::chrono::system_clock::time_point tp = 
                                     std::chrono::system_clock::from_time_t(tt);
    // Add 0.123456 seconds
    // This will not compile if std::chrono::system_clock::time_point has
    //   courser resolution than microseconds
    tp += std::chrono::microseconds(123456);
    
    // Now output tp

    // Convert std::chrono::system_clock::time_point to std::time_t
    tt = std::chrono::system_clock::to_time_t(tp);
    // Convert std::time_t to std::tm (popular extension)
    tm = std::tm{0};
    gmtime_r(&tt, &tm);
    // Output month
    std::cout << tm.tm_mon + 1 << '-';
    // Output day
    std::cout << tm.tm_mday << '-';
    // Output year
    std::cout << tm.tm_year+1900 << ' ';
    // Output hour
    if (tm.tm_hour <= 9)
        std::cout << '0';
    std::cout << tm.tm_hour << ':';
    // Output minute
    if (tm.tm_min <= 9)
        std::cout << '0';
    std::cout << tm.tm_min << ':';
    // Output seconds with fraction
    //   This is the heart of the question/answer.
    //   First create a double-based second
    std::chrono::duration sec = tp - 
                                    std::chrono::system_clock::from_time_t(tt) +
                                    std::chrono::seconds(tm.tm_sec);
    //   Then print out that double using whatever format you prefer.
    if (sec.count() < 10)
        std::cout << '0';
    std::cout << std::fixed << sec.count() << '\n';
}

For me this outputs:

10-10-2012 12:38:40.123456

Your std::chrono::system_clock::time_point may or may not be precise enough to hold microseconds.

Update

An easier way is to just use this date library. The code simplifies down to (using C++14 duration literals):

#include "date.h"
#include 
#include 

int
main()
{
    using namespace date;
    using namespace std::chrono;
    auto t = sys_days{10_d/10/2012} + 12h + 38min + 40s + 123456us;
    static_assert(std::is_same>{}, "");
    std::cout << t << '\n';
}

which outputs:

2012-10-10 12:38:40.123456

You can skip the static_assert if you don't need to prove that the type of t is a std::chrono::time_point.

If the output isn't to your liking, for example you would really like dd-mm-yyyy ordering, you could:

#include "date.h"
#include 
#include 

int
main()
{
    using namespace date;
    using namespace std::chrono;
    using namespace std;
    auto t = sys_days{10_d/10/2012} + 12h + 38min + 40s + 123456us;
    auto dp = floor(t);
    auto time = make_time(t-dp);
    auto ymd = year_month_day{dp};
    cout.fill('0');
    cout << ymd.day() << '-' << setw(2) << static_cast(ymd.month())
         << '-' << ymd.year() << ' ' << time << '\n';
}

which gives exactly the requested output:

10-10-2012 12:38:40.123456

Update

Here is how to neatly format the current time UTC with milliseconds precision:

#include "date.h"
#include 

int
main()
{
    using namespace std::chrono;
    std::cout << date::format("%F %T\n", time_point_cast(system_clock::now()));
}

which just output for me:

2016-10-17 16:36:02.975

C++17 will allow you to replace time_point_cast with floor. Until then date::floor is available in "date.h".

std::cout << date::format("%F %T\n", date::floor(system_clock::now()));

Update C++20

In C++20 this is now simply:

#include 
#include 

int
main()
{
    using namespace std::chrono;
    auto t = sys_days{10d/10/2012} + 12h + 38min + 40s + 123456us;
    std::cout << t << '\n';
}

Or just:

std::cout << std::chrono::system_clock::now() << '\n';

std::format will be available to customize the output.