How to convert a timestamp from std::chrono::nanoseconds to ros::Time ?

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The ros::Time object is made from seconds and nanoseconds, as mentioned here. Therefore, we need to extract these values (sec and nsec) from std::chrono. See an example below:

const auto p0 = std::chrono::time_point<std::chrono::high_resolution_clock>{};
const auto p3 = std::chrono::high_resolution_clock::now();

auto tstamp = p3 - p0;
int32_t sec = std::chrono::duration_cast<std::chrono::seconds>(tstamp).count();
int32_t nsec = std::chrono::duration_cast<std::chrono::nanoseconds>(tstamp).count() % 1000000000UL;
std::cout << "sec: " << sec << " nsec: " << nsec << " since epoch: \n";
ros::Time n(sec, nsec);

Please note that the above code is taken from here.

Furthermore, you can see the API that ros::Time can be made seconds only. Here seconds are represented by a double datatype. Please see here and here also for more info. In short, you just need to convert nanoseconds i.e., std::chrono::nanoseconds to seconds and use it as ros::Time n(sec).