C++ shares its date/time functions with C. The tm structure is probably the easiest for a C++ programmer to work with - the following prints today's date:

#include <ctime>
#include <iostream>

int main() {
    std::time_t t = std::time(0);   // get time now
    std::tm* now = std::localtime(&t);
    std::cout << (now->tm_year + 1900) << '-' 
         << (now->tm_mon + 1) << '-'
         <<  now->tm_mday
         << "\n";
}

std C libraries provide time(). This is seconds from the epoch and can be converted to date and H:M:S using standard C functions. Boost also has a time/date library that you can check.

time_t  timev;
time(&timev);

You can use the following code to get the current system date and time in C++ :

    #include <iostream>
    #include <time.h> //It may be #include <ctime> or any other header file depending upon
                     // compiler or IDE you're using 
    using namespace std;

    int main() {
       // current date/time based on current system
       time_t now = time(0);

       // convert now to string form
       string dt = ctime(&now);

       cout << "The local date and time is: " << dt << endl;
    return 0;
    }

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std::ctime

Why was ctime only mentioned in the comments so far?

#include <ctime>
#include <iostream>
 
int main()
{
    std::time_t result = std::time(nullptr);
    std::cout << std::ctime(&result);
}

Output

Tue Dec 27 17:21:29 2011

#include <iostream>
#include <chrono>
#include <string>
#pragma warning(disable: 4996)
// Ver: C++ 17 
// IDE: Visual Studio
int main() {
    using namespace std; 
    using namespace chrono;
    time_point tp = system_clock::now();
    time_t tt = system_clock::to_time_t(tp);
    cout << "Current time: " << ctime(&tt) << endl;
    return 0;
}

Here is the non-deprecated modern C++ solution for getting a timestamp as a std::string for use with e.g. filenames:

std::string get_file_timestamp()
{
    const auto now = std::chrono::system_clock::now();
    const auto in_time_t = std::chrono::system_clock::to_time_t(now);

    std::stringstream output_stream;

    struct tm time_info;
    const auto errno_value = localtime_s(&time_info, &in_time_t);
    if(errno_value != 0)
    {
        throw std::runtime_error("localtime_s() failed: " + std::to_string(errno_value));
    }

    output_stream << std::put_time(&time_info, "%Y-%m-%d.%H_%M_%S");
    return output_stream.str();
}

I needed a way to insert current date-time at every update of a list. This seems to work well, simply.

#include<bits/stdc++.h>
#include<unistd.h>
using namespace std;
int main()
{   //initialize variables
    time_t now; 
    //blah..blah
    /*each time I want the updated stamp*/
    now=time(0);cout<<ctime(&now)<<"blah_blah";
}