Create std::chrono::zoned_time from zone and time

Question

I have a datetime in a platonic sense, i.e some date and time (like 18th of January 2022 15:15:00) and I know in which timezone it represent something, e.g "Europe/Moscow"

I want to create std::chrono::zoned_time. Is is possible? I looked at the constructors and it seems all of them require either sys_time or local_time which is not what I have.

Am I missing something obvious?

Answer 1

#include <chrono>
#include <iostream>

int
main()
{
    using namespace std::literals;
    std::chrono::zoned_time zt{"Europe/Moscow",
        std::chrono::local_days{18d/std::chrono::January/2022} + 15h + 15min};
    std::cout << zt << '\n';
}

local_time isn't necessarily the computer's local time. It is a local time that has not yet been associated with a time zone. When you construct a zoned_time, you associate a local time with a time zone.

The above program prints out:

2022-01-18 15:15:00 MSK

So you can think of this as the identity function. But in reality you can also get the UTC time out of zt with .get_sys_time(). And you can also use zt as the "time point" to construct another zoned_time:

std::chrono::zoned_time zt2{"America/New_York", zt};
std::cout << zt2 << '\n';

Output:

2022-01-18 07:15:00 EST

zt2 will have the same sys_time (UTC) as zt. This makes it handy to set up international video conferences (for example).