C++生成13位時間戳

#include <string>
#include <stdio.h>


string GetUnixTime()
{
    string nowTimeUnix;
    string cs_uninxtime;
    string cs_milliseconds;
    SYSTEMTIME sysTime;
    GetLocalTime(&sysTime);
    time_t unixTime;
    time(&unixTime);
    char buf[30],buf1[30];
    sprintf_s(buf,sizeof(buf),"%I64d", (INT64)unixTime);
    sprintf_s(buf1,sizeof(buf1),"%I64d", (INT64)sysTime.wMilliseconds);
    nowTimeUnix =string(buf) + string(buf1);
    return nowTimeUnix;
}