Various ways to print date time

date_format.cpp

static std::string date_format(boost::posix_time::ptime const& datetime) {
    using namespace boost;
    std::ostringstream ss;

    auto output_facet = new boost::posix_time::time_facet();
    ss.imbue(std::locale(std::locale::classic(), output_facet));
    output_facet->format("%Y-%m-%d %H:%M:%s%Q");

    ss.str("");
    ss << datetime;
    return ss.str();
}

static std::string date_format2() {

    namespace bg = boost::gregorian;

    static char const* const fmt = "%Y-%m-%d %H:%M:%S%Q";
    std::ostringstream ss;
    // assumes std::cout's locale has been set appropriately for the entire app
    ss.imbue(std::locale(std::cout.getloc(), new bg::date_facet(fmt)));
    ss << bg::day_clock::universal_day();
    return ss.str();
}

static std::string date_format3() {
    auto now = std::chrono::system_clock::now();
    auto in_time_t = std::chrono::system_clock::to_time_t(now);

    std::stringstream ss;
    ss << std::put_time(std::localtime(&in_time_t), "%Y-%m-%d %H:%M:%S%.000%Ez");
    return ss.str();
}

static boost::posix_time::time_duration get_utc_offset() {
    using namespace boost::posix_time;

    // boost::date_time::c_local_adjustor uses the C-API to adjust a
    // moment given in utc to the same moment in the local time zone.
    typedef boost::date_time::c_local_adjustor<ptime> local_adj;

    const ptime utc_now = second_clock::universal_time();
    const ptime now = local_adj::utc_to_local(utc_now);

    return now - utc_now;
}

static std::string get_utc_offset_string() {
    std::stringstream out;

    using namespace boost::posix_time;
    auto tf = new time_facet();
    tf->time_duration_format("%+%H:%M");
    out.imbue(std::locale(out.getloc(), tf));

    out << get_utc_offset();

    return out.str();
}

static std::string date_format4() {
    auto now = std::chrono::system_clock::now();
    auto in_time_t = std::chrono::system_clock::to_time_t(now);

    std::stringstream ss;
    ss << std::put_time(std::localtime(&in_time_t), "%Y-%m-%d %H:%M:%S%.000") << get_utc_offset_string();
    return ss.str();
}

static std::string date_format5() {
    char output[200];
    const char* fmt = "%Y-%m-%d %H:%M:%S%.000";
    auto t = time(nullptr);
    auto tmp = gmtime(&t);
    if (tmp == nullptr) {
        throw std::logic_error("gmtime error");
    }

    if (strftime(output, sizeof(output), fmt, tmp) == 0) {
        throw std::logic_error("strftime returned 0");
    }

    return std::string(output) + get_utc_offset_string(); //"+00:00";
}