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How to convert std::chrono::time_point to milliseconds since epoch

If you have a std::chrono::time_point such as

timepoint_now.cpp
std::chrono::time_point<std::chrono::system_clock> timepoint = std::chrono::system_clock::now();

You can convert it to milliseconds since epoch by first casting it to a chrono::duration using

timepoint_time_since_epoch.cpp
timepoint.time_since_epoch(); // Returns a duration

and then using std::chrono::duration_cast<std::chrono::milliseconds>(...).count() to convert it to milliseconds since epoch and getting the number of intervals (= seconds) in between epoch and the time point.

timepoint_to_ms.cpp
std::chrono::duration_cast<std::chrono::milliseconds>(timepoint.time_since_epoch()).count()

Full example:

timepoint_example.cpp
#include <chrono>
#include <iostream>

int main()
{
    auto timepoint = std::chrono::system_clock::now();
    std::cout << "Milliseconds since epoch: " <<
        std::chrono::duration_cast<std::chrono::milliseconds>(timepoint.time_since_epoch()).count() << std::endl;
}

This prints, for example:

ms_output.txt
Milliseconds since epoch: 1690755347529

 

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