Get current time in milliseconds, or HH:MM:SS:MMM format

This is a portable method using the C++11 chrono library:

#include <chrono>
#include <ctime>
#include <iomanip>
#include <sstream>
#include <string>

// ...

std::string time_in_HH_MM_SS_MMM()
{
    using namespace std::chrono;

    // get current time
    auto now = system_clock::now();

    // get number of milliseconds for the current second
    // (remainder after division into seconds)
    auto ms = duration_cast<milliseconds>(now.time_since_epoch()) % 1000;

    // convert to std::time_t in order to convert to std::tm (broken time)
    auto timer = system_clock::to_time_t(now);

    // convert to broken time
    std::tm bt = *std::localtime(&timer);

    std::ostringstream oss;

    oss << std::put_time(&bt, "%H:%M:%S"); // HH:MM:SS
    oss << '.' << std::setfill('0') << std::setw(3) << ms.count();

    return oss.str();
}

This is a cleaner solution using HowardHinnant's date library.

std::string get_time()
{
    using namespace std::chrono;
    auto now = time_point_cast<milliseconds>(system_clock::now());
    return date::format("%T", now);
}