Working with Dates and Time in C++: A Complete Guide to chrono

Working with dates and time in C++: from <ctime> to modern <chrono>

Working with dates and time is one of the key tasks in any serious application: from logs and backups to timers and financial calculations. In C++, two approaches have historically developed: the classic C-style through the <ctime> header file and the modern, type-safe approach through the <chrono> library, which appeared in C++11 and was significantly expanded in C++20. In this article, we will cover both methods, learn how to get the current time, measure intervals, format dates, and avoid common mistakes.

1. Classic approach: <ctime> and std::time_t

Before C++11, the only standard way to work with time was C functions: time(), localtime(), strftime(). This approach is still used in legacy code and simple tasks.

Getting the current time

The std::time(nullptr) function returns the number of seconds since January 1, 1970 (Unix epoch) as an integer std::time_t.

#include <iostream>#include <ctime>

int main() {    std::time_t now = std::time(nullptr);    std::cout << "Current time (seconds since epoch): " << now << std::endl;    return 0;}

Converting to a readable format

To get a structure with components (year, month, day, hours, etc.), use std::localtime() or std::gmtime().

#include <iostream>#include <ctime>

int main() {    std::time_t now = std::time(nullptr);    std::tm* local = std::localtime(&now);

    std::cout << "Year: " << (local->tm_year + 1900) << std::endl;    std::cout << "Month: " << (local->tm_mon + 1) << std::endl;    std::cout << "Day: " << local->tm_mday << std::endl;    return 0;}

Formatting a string with strftime

The std::strftime() function allows you to flexibly format a date into a string according to a given template.

#include <iostream>#include <ctime>

int main() {    std::time_t now = std::time(nullptr);    std::tm* local = std::localtime(&now);

    char buffer[80];    std::strftime(buffer, sizeof(buffer), "%d.%m.%Y %H:%M:%S", local);    std::cout << "Current date and time: " << buffer << std::endl;    return 0;}

⚠️ Important: std::localtime() and std::gmtime() return a pointer to a static buffer — this is not thread-safe. In C++11, reentrant versions localtime_s (Windows) or localtime_r (POSIX) appeared.

2. Modern C++: the <chrono> library

The <chrono> library offers strong typing, work with different time units, and high precision. Main components:

• Clocks: system_clock, steady_clock, high_resolution_clock
• Time points: binding to a specific moment
• Durations: time intervals (seconds, milliseconds, etc.)

Getting the current time point

#include <iostream>#include <chrono>#include <ctime> // for std::time_t

int main() {    auto now = std::chrono::system_clock::now();    std::time_t now_time = std::chrono::system_clock::to_time_t(now);    std::cout << "Current time: " << std::ctime(&now_time);    return 0;}

Measuring time intervals (timer)

One of the most common tasks is to measure code execution time. For this, it is perfect to use