长话短说：不要重新发明轮子，只需使用std::chrono库中可用的year_month_day std::formatter和hh_mm_ss std::formatter。

#include <chrono>
#include <format>
#include <iostream>
#include <string>

static std::string GetTimestamp(bool fourDigitYear) {
    const auto now = std::chrono::system_clock::now();
    std::chrono::year_month_day ymd{ std::chrono::floor<std::chrono::days>(now) };
    std::chrono::zoned_time zt{ std::chrono::current_zone(), (now) };
    const auto lt = zt.get_local_time();
    const auto tod = lt - std::chrono::floor<std::chrono::days>(lt);
    const std::chrono::hh_mm_ss hms{ std::chrono::floor<std::chrono::seconds>(tod) };

    if (fourDigitYear) {
        return std::format("Y{0:%Y}M{0:%m}D{0:%d}_H{1:%H}M{1:%M}S{1:%S%p}", ymd, hms);
    }
    else {
        return std::format("Y{0:%y}M{0:%m}D{0:%d}_H{1:%H}M{1:%M}S{1:%S%p}", ymd, hms);
    }
}

int main() {
    std::cout << GetTimestamp(true);
}

字符串
为了好玩，这里有一个模板化的if-consexpr版本：

#include <chrono>
#include <format>
#include <iostream>
#include <string>

template<typename bool fourDigitYear>
static std::string GetTimestamp() {
    const auto now = std::chrono::system_clock::now();
    const std::chrono::year_month_day ymd{ std::chrono::floor<std::chrono::days>(now) };
    const std::chrono::zoned_time zt{ std::chrono::current_zone(), (now) };
    const auto lt = zt.get_local_time();
    const auto tod = lt - std::chrono::floor<std::chrono::days>(lt);
    const std::chrono::hh_mm_ss hms{ std::chrono::floor<std::chrono::seconds>(tod) };

    if constexpr (fourDigitYear) {
        return std::format("Y{0:%Y}M{0:%m}D{0:%d}_H{1:%H}M{1:%M}S{1:%S%p}", ymd, hms);
    } else {
        return std::format("Y{0:%y}M{0:%m}D{0:%d}_H{1:%H}M{1:%M}S{1:%S%p}", ymd, hms);
    }
}

int main() {
    std::cout << GetTimestamp<false>() << '\n';
    std::cout << GetTimestamp<true>();
}

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