tm结构体的定义在time.h里面

struct tm {
    int tm_sec;
    int tm_min;
    int tm_hour;
    int tm_mday;
    int tm_mon;
    int tm_year;
    int tm_wday;
    int tm_yday;
    int tm_isdst;

};

/*
 *  linux/kernel/mktime.c
 *
 *  (C) 1991  Linus Torvalds
 */

#include <time.h>

/*
 * This isn't the library routine, it is only used in the kernel.
 * as such, we don't care about years<1970 etc, but assume everything
 * is ok. Similarly, TZ etc is happily ignored. We just do everything
 * as easily as possible. Let's find something public for the library
 * routines (although I think minix times is public).
 */
/*
 * PS. I hate whoever though up the year 1970 - couldn't they have gotten
 * a leap-year instead? I also hate Gregorius, pope or no. I'm grumpy.
 */
#define MINUTE 60         //一分钟60秒，这里60是以秒为单位计数
#define HOUR (60*MINUTE)  //一小时60分钟
#define DAY (24*HOUR)     //一天24小时
#define YEAR (365*DAY)    //一年365天

/* interestingly, we assume leap-years */
static int month[12] = {//初始化每个月开始的秒数，即以秒为单位的起始时间
    0,
    DAY*(31),
    DAY*(31+29),
    DAY*(31+29+31),
    DAY*(31+29+31+30),
    DAY*(31+29+31+30+31),
    DAY*(31+29+31+30+31+30),
    DAY*(31+29+31+30+31+30+31),
    DAY*(31+29+31+30+31+30+31+31),
    DAY*(31+29+31+30+31+30+31+31+30),
    DAY*(31+29+31+30+31+30+31+31+30+31),
    DAY*(31+29+31+30+31+30+31+31+30+31+30)
};

long kernel_mktime(struct tm * tm)
{
    long res;
    int year;

    year = tm->tm_year - 70;//从1970年开始计时，year得到的是和70年的年差
/* magic offsets (y+1) needed to get leapyears right.*/
    res = YEAR*year + DAY*((year+1)/4);//闰年多一天
    res += month[tm->tm_mon];//时间精确到月
/* and (y+2) here. If it wasn't a leap-year, we have to adjust */
    if (tm->tm_mon>1 && ((year+2)%4))
    //因为是从1970年算起的，这里year+2就能耦合上闰年。。。换而言之，1972年是闰年，但是年差是2，这里补上2就对齐了。。。。
        res -= DAY;//year不是闰年，则减一天
    res += DAY*(tm->tm_mday-1);
    res += HOUR*tm->tm_hour;
    res += MINUTE*tm->tm_min;
    res += tm->tm_sec;//时间精确到秒
    return res;
}