How to Print System Time in C++

1. Use std::chrono::system_clock and std::ctime to Print System Time in C++
2. Use time, localtime and strftime to Print System Time in C++

This article will explain several methods of how to print system time in C++.

Use std::chrono::system_clock and std::ctime to Print System Time in C++

std::chrono::system_clock represents the system-wide wall clock and it provides two functions to convert to/from std::time_t type. The latter object we can process using the ctime function and return a null-terminated string of the form - Wed Jun 30 21:49:08 1993\n. In this case, we construct a separate function to encapsulate both calls and return the string value to the caller. Notice that we also remove the new line character to return value in a more flexible form. Also, system_clock::now is used to retrieve the current timepoint.

#include <sys/time.h>

#include <chrono>
#include <ctime>
#include <iostream>

using std::cout;
using std::endl;
using std::string;

std::string timeToString(std::chrono::system_clock::time_point &t) {
  std::time_t time = std::chrono::system_clock::to_time_t(t);
  std::string time_str = std::ctime(&time);
  time_str.resize(time_str.size() - 1);
  return time_str;
}

int main() {
  auto time_p = std::chrono::system_clock::now();
  cout << "Current time: " << timeToString(time_p) << endl;

  return EXIT_SUCCESS;
}

Output:

Current time: Fri Apr  1 01:34:20 2021

One can also utilize the same method to display Epoch value in a similar form, as demonstrated in the following example. Note that, Epoch is usually January 1 of 1970 for POSIX/UNIX systems, but it’s not required to be the same value for different clocks provided in the chrono library.

#include <sys/time.h>

#include <chrono>
#include <ctime>
#include <iostream>

using std::cout;
using std::endl;
using std::string;

std::string timeToString(std::chrono::system_clock::time_point &t) {
  std::time_t time = std::chrono::system_clock::to_time_t(t);
  std::string time_str = std::ctime(&time);
  time_str.resize(time_str.size() - 1);
  return time_str;
}

int main() {
  std::chrono::system_clock::time_point time_p2;
  cout << "Epoch: " << timeToString(time_p2) << endl;

  return EXIT_SUCCESS;
}

Output:

Epoch: Thu Jan  1 00:00:00 1970

Use time, localtime and strftime to Print System Time in C++

Alternatively, we can use POSIX specific function - time and directly retrieve the time_t structure. time_t is essentially an integer that stores the number of seconds since the Epoch. Similarly to the previous method, we can use ctime to convert to the string of predefined form or call the localtime function. The localtime function converts the time_t object to the tm structure, which is the broken-down time format that can be utilized to format the output string as we wish with special specifiers. Formatting is done by the strftime function that takes four parameters, the last of which is the pointer to struct tm. The first argument specifies the memory address where the character string will be stored, and the next two arguments are the maximum size of the string and the format specifier. The detailed overview of format specification can be seen on this page.

#include <sys/time.h>

#include <ctime>
#include <iostream>

using std::cout;
using std::endl;
using std::string;

int main() {
  char tt[100];
  time_t now = time(nullptr);
  auto tm_info = localtime(&now);

  strftime(tt, 100, "%Y-%m-%d %H:%M:%S", tm_info);
  puts(tt);

  return EXIT_SUCCESS;
}

Output:

2021-04-02 05:42:46