Get Unix timestamp with C++
How do I get a uint
unix timestamp in C++? I've googled a bit and it seems that most methods are looking for more convoluted ways to represent time. Can't I just get it as a uint
?
Solution 1:
C++20 introduced a guarantee that time_since_epoch
is relative to the UNIX epoch, and cppreference.com gives an example that I've distilled to the relevant code, and changed to units of seconds rather than hours:
#include <iostream>
#include <chrono>
int main()
{
const auto p1 = std::chrono::system_clock::now();
std::cout << "seconds since epoch: "
<< std::chrono::duration_cast<std::chrono::seconds>(
p1.time_since_epoch()).count() << '\n';
}
Using C++17 or earlier, time()
is the simplest function - seconds since Epoch, which for Linux and UNIX at least would be the UNIX epoch. Linux manpage here.
The cppreference page linked above gives this example:
#include <ctime>
#include <iostream>
int main()
{
std::time_t result = std::time(nullptr);
std::cout << std::asctime(std::localtime(&result))
<< result << " seconds since the Epoch\n";
}
Solution 2:
#include<iostream>
#include<ctime>
int main()
{
std::time_t t = std::time(0); // t is an integer type
std::cout << t << " seconds since 01-Jan-1970\n";
return 0;
}
Solution 3:
The most common advice is wrong, you can't just rely on time()
. That's used for relative timing: ISO C++ doesn't specify that 1970-01-01T00:00Z
is time_t(0)
What's worse is that you can't easily figure it out, either. Sure, you can find the calendar date of time_t(0)
with gmtime
, but what are you going to do if that's 2000-01-01T00:00Z
? How many seconds were there between 1970-01-01T00:00Z
and 2000-01-01T00:00Z
? It's certainly no multiple of 60, due to leap seconds.
Solution 4:
As this is the first result on google and there's no C++20 answer yet, here's how to use std::chrono to do this:
#include <chrono>
//...
using namespace std::chrono;
int64_t timestamp = duration_cast<milliseconds>(system_clock::now().time_since_epoch()).count();
In versions of C++ before 20, system_clock's epoch being Unix epoch is a de-facto convention, but it's not standardized. If you're not on C++20, use at your own risk.