I'm doing a bit of guessing with this answer, but my guess is that you want to truncate the current time to the floor of the current half minute (in case of iRoundTo == 30).

If I'm correct, this is easy to do as long as iRoundTo is a compile-time constant.

#include <chrono>
#include <iostream>

int
main()
{
    using RoundTo = std::chrono::duration<int, std::ratio<30>>;
    auto Now = std::chrono::system_clock::now();
    auto dt = std::chrono::floor<RoundTo>(Now);
    std::cout << dt << '\n';
}

The above creates a new duration type that is 30 seconds long. It then gets the current time and floors it (truncates downwards) to the previous half minute unit. It then prints out the result. Everything before the print works in C++17. The printing (the last line) requires C++20.

Example output for me:

2022-01-18 01:45:30

The above code can also work pre-C++17 but in that case you'll need to find your own floor (e.g. date.h) or use std::chrono::duration_cast which truncates towards zero.

Update

In the comments below, it is explained that iRoundTo is not a compile-time constant, but does always represent a multiple of seconds. In this case I would do this in two steps:

int iRoundTo = 30;
auto Now = std::chrono::system_clock::now();

// First truncate to seconds
auto dt = std::chrono::floor<std::chrono::seconds>(Now);
// Then subtract of the modulus according to iRoundTo
dt -= dt.time_since_epoch() % iRoundTo;

The sub-expression dt.time_since_epoch() % iRoundTo has type seconds and a value between 0s and seconds{iRoundTo} - 1s.

Without access to the newest tools (and both std::format and << overload seem to require rather new tools) I've got got it tested only so far (please edit if you can test it further):

#include <chrono>
#include <iostream>
#include <ctime>
#include <iomanip>
//#include <format>

int main() {
    int roundTo = 15;
    auto dt = std::chrono::floor<std::chrono::seconds> (std::chrono::system_clock::now());
    dt -= dt.time_since_epoch() % roundTo;
    std::time_t t = std::chrono::system_clock::to_time_t(dt);
    std::cout << std::ctime(&t); // Using <ctime>
    std::cout << std::put_time(std::localtime(&t), "%Y-%m-%e %H:%M:%S") << '\n'; // Using <iomanip>
    //std::cout << "date: " << std::format("%Y-%m-%e %H:%M:%S", dt) << '\n'; // Using <format>
    //std::cout << "date: " << std::format("{%Y-%m-%e %H:%M:%S}", dt) << '\n'; // Using newer <format>
    //std::cout << dt << '\n'; // Using << overload
}