What is the correct way to get beginning of the day in UTC / GMT?

Question

::tm tm{0, 0, 0, 29, 10, 2022 - 1900, 0, 0};  // 10 for November
auto time_t = ::mktime(&tm);
cout << "milliseconds = " << time_t * 1000 << endl;

Above code outputs 1669660200000, which is equivalent to 2022 November 29, 00:00:00. But it is in local timezone. How to get the UTC time for the aforementioned date?
A modern c++17 way with thread-safety will be appreciated.

Answer 1

There's a nit picky weak point in your solution (besides the thread safety issue): The members of tm are not guaranteed to be in the order you are assuming.

The tm structure shall contain at least the following members, in any order.

Using C++17 you can use this C++20 chrono preview library. It is free, open-source and header-only. Your program would look like:

#include "date/date.h"
#include <chrono>
#include <iostream>

int
main()
{
    using namespace std;
    using namespace chrono;
    using namespace date;

    sys_time<milliseconds> tp = sys_days{2022_y/11/29};
    cout << "milliseconds = " << tp.time_since_epoch().count() << '\n';
}

And the output would be:

milliseconds = 1669680000000

One of the nice advantages of using this library is that it will easily port to C++20. The C++20 version looks like:

#include <chrono>
#include <iostream>

int
main()
{
    using namespace std;
    using namespace chrono;

    sys_time<milliseconds> tp = sys_days{2022y/11/29};
    cout << "milliseconds = " << tp.time_since_epoch() << '\n';
}

And outputs:

milliseconds = 1669680000000ms

Demo: