How can I round Time epoch to previous midnight - c++

Question

Let's say I have :
time in msec = 1655205146419 , which is generated using :
auto msec = duration_cast<milliseconds>(system_clock::now().time_since_epoch()).count();
Tuesday, June 14, 2022 11:12:26.419 AM
And I want to round it up to the previous midnight :
Tuesday, June 14, 2022 00:00:00.00 AM
any ideas how ?

Answer 1

You can use https://en.cppreference.com/w/cpp/chrono/time_point/floor with a duration https://en.cppreference.com/w/cpp/chrono/duration of 1 day:
#include <iostream>
#include <chrono>
int main()
{
using Day = std::chrono::days;
using std::chrono::duration_cast;
auto start = std::chrono::system_clock::now();
std::cout << duration_cast<std::chrono::milliseconds>(std::chrono::floor<Day>(start).time_since_epoch()).count() << "\n";
}
https://godbolt.org/z/z89nr4Mo3
increased readability when storing the days in a variable:
auto start = std::chrono::system_clock::now();
auto days = std::chrono::floor<Day>(start).time_since_epoch();
std::cout << duration_cast<std::chrono::milliseconds>(days) << std::endl;

Answer 2

Let's say I have : time in msec = 1655205146419
I'd try to stay within the chrono domain as much as it is possible, but if you are forced to use milliseconds since the epoch as an input value:
auto midnight_tp = system_clock::time_point(milliseconds(msec)) -
milliseconds(msec) % days(1);
// or
auto midnight_tp = floor<days>(system_clock::time_point(milliseconds(msec)));
Getting to the unitless milliseconds since the epoch again is just a repetition what you've already done earlier:
auto midnight_msec =
duration_cast<milliseconds>(midnight_tp.time_since_epoch()).count();
Demo

Answer 3

You could do it this way, not as pretty, but you can play a bit with std::tm and get answers that you wish.
#include <ctime>
#include <iostream>
#include <string>
int main() {
std::time_t now = time(0);
std::tm tstruct;
char current_time[20];
char last_midnight[20];
tstruct = *std::localtime(&now);
std::strftime(current_time, sizeof(current_time), "%Y-%m-%d %X", &tstruct);
tstruct.tm_hour = 00;
tstruct.tm_min = 00;
tstruct.tm_sec = 00;
std::strftime(last_midnight, sizeof(last_midnight), "%Y-%m-%d %X", &tstruct);
std::cout << "current_time: " << current_time << std::endl;
std::cout << "last_midnight: " << last_midnight << std::endl;
return 0;
}
Output results