使用Howard Hinnant的date.h计算今年日期的天数

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

int main() {
    // 获取当前日期
    auto today = date::year_month_day(date::floor<date::days>(std::chrono::system_clock::now()));

    // 获取今年的第一天
    auto this_year = date::year_month_day{today.year(), date::January, 1};

    // 计算天数差
    auto days = (today - this_year).count() + 1;

    std::cout << "今年的天数：" << days << std::endl;

    return 0;
}