Unix epoch seconds corresponding to midnight in a local time zone

Question

I came up with the following function to obtain epoch seconds that correspond to previous midnight in a local time zone:

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

long local_midnight_epoch() {
    using namespace date;
    using namespace std::chrono;
    auto t = std::chrono::system_clock::now();
    auto* zone = date::current_zone();
    auto zt = make_zoned(zone, t);
    zt = floor<days>(zt.get_local_time());
    auto seconds_since_midnight = floor<seconds>(t - zt.get_sys_time());
    std::chrono::time_point<std::chrono::system_clock> ts = std::chrono::system_clock::now();
    auto epoch = std::chrono::duration_cast<std::chrono::seconds>(ts.time_since_epoch());
    epoch -= seconds_since_midnight;
    return epoch.count();
}

int main(void) {
    std::cerr << local_midnight_epoch() << std::endl;
}

The code uses Howard Hinnant's date library. It first computes seconds passed since midnight in a local time zone, and then subtract that the current epoch. Is there an easier way to accomplish this? Thanks.

Answer 1

This is a good attempt at getting the UTC timestamp associated with the previous local midnight. For me it gets within a second or two of the correct answer.

Here's what I recommend, with each line commented with what it does:

long local_midnight_epoch2() {
    using namespace date;
    using namespace std::chrono;
    // current time in seconds, UTC
    auto now = floor<seconds>(system_clock::now());
    // corresponding local time
    zoned_seconds zt{current_zone(), now};
    // reset to previous local midnight
    zt = floor<days>(zt.get_local_time());
    // get UTC corresponding to local midnight
    auto epoch = zt.get_sys_time();
    return epoch.time_since_epoch().count();
}

The biggest advantage over what you have is that it only calls system_clock::now() once. Otherwise it is quite similar to what you have.