一道C++算法题：将纳秒数转化为日期时间

问题

通过给定当前时间相对于1970/01/01 00:00:00 的纳秒数，求当前的时间。比如当前纳秒数为 1597792400289124531，对应的时间为2020-08-19 07:13:20.891

算法

利用C++11提供的纳秒获取函数求得当前纳秒数，然后除以1000000000求得秒，在计算时考虑闰年，然后计算求出相应的年月日时分秒。注意在计算的时候还要考虑时区，比如中国在东8区，还要加上8小时的秒数。

实现代码

#include 
#include 
#include 
#include 

using namespace std;
using namespace std::chrono;

void getDate(long long time) {
     
    int seconds = time / 1000000000 + 8 * 3600;// time zone +8
    int days = seconds / 86400;
    int year = 1970 + (int) (days / 1461) * 4; // recycled in 4 years = 1461 days
    int month = 0;
    int day = days % 1461;
    day = day > 730 ? day - 1 : day;
    year += (int) (day / 365);
    day = day % 365;
    int monthday[]{
     0, 31, 59, 90, 120, 151, 181, 212, 243, 273, 304, 334, 365};
    for (int i = 0; i < 13; i++) {
     
        if (day < monthday[i + 1]) {
     
            month = i + 1;
            day = day - monthday[i] + 1;
            break;
        }
    }
    int sec = seconds % 86400;
    int hour = (int) (sec / 3600);
    int minute = (int) (sec % 3600 / 60);
    int second = sec % 60;
    int nanoseconds = time % 1000000000;
    int millisecond = nanoseconds / 1000;

    cout << year << "/" << month << "/" << day << " " << hour << ":"
         << minute << ":" << second << "." << millisecond << endl;
}

int main() {
     
    // 获取当前时间
    system_clock::time_point now = system_clock::now();
    // 距离1970-01-01 00:00:00的纳秒数
    chrono::nanoseconds d = now.time_since_epoch();
    cout << "current nanoseconds:" << d.count() << endl <<  "currnet date time:  " ;
    getDate(d.count()) ;
    return 0;
}

输出

current nanoseconds:1597793495969797782
currnet date time:  2020/8/19 7:31:35.969797