#include <chrono>
#include <string>
#include <iostream>
#include <iomanip>

std::string get_format_time()
{
    auto now = std::chrono::system_clock::now();
    auto timet = std::chrono::system_clock::to_time_t(now);
    //std::cout << "timet: " << timet << std::endl; //timet: 1637235226
    auto localTime = *std::gmtime(&timet);

    auto ms = std::chrono::time_point_cast<std::chrono::milliseconds>(now).time_since_epoch().count() % 1000;
    //std::cout << "ms: " << ms << std::endl; //ms: 632

    std::stringstream ss; 
    std::string str;
    ss << std::put_time(&localTime, "%Y-%m-%dT%H:%M:%S") << "." << std::setfill('0') << std::setw(3) << ms;
    ss >> str;

    return str;
}

int main()
{
    std::cout << get_format_time() << std::endl;
    // 输出：2021-11-18T13:45:49.047

    return 0;
}

#include <chrono>
#include <string>
#include <iostream>
#include <iomanip>

std::string format_timestamp(int timestamp)
{

    auto sec = std::chrono::seconds(timestamp);
    std::chrono::time_point<std::chrono::system_clock> now(sec);

    auto timet = std::chrono::system_clock::to_time_t(now);
    auto localTime = *std::gmtime(&timet);

    std::stringstream ss;
    std::string str;
    ss << std::put_time(&localTime, "%Y-%m-%dT%H:%M:%S");
    ss >> str;

    return str;
}

int main()
{
    std::cout << format_timestamp(1630238012) << std::endl;
    // 输出：2021-08-29T11:53:32

    return 0;
}