谁帮忙解析一下这个计算时间的函数

[ewolfe163]

nginx中, ngx_parse_time.c中函数中有如下语句：
/* days before the month */
+ 367 * month / 12 - 30
这句注释是计算month前的天数，介不明白是如何实现的？不明白这里为什么是367呢？ 为什么-30?

函数源码：：

static ngx_uint_t mday[] = { 31, 28, 31, 30, 31, 30, 31, 31, 30, 31, 30, 31 };

time_t
ngx_parse_http_time(u_char *value, size_t len)
{
u_char *p, *end;
ngx_int_t month;
ngx_uint_t day, year, hour, min, sec;
uint64_t time；
enum {
no = 0,
rfc822, /* Tue, 10 Nov 2002 23:50:13 */
rfc850, /* Tuesday, 10-Dec-02 23:50:13 */
isoc /* Tue Dec 10 23:50:13 2002 */
} fmt;

fmt = 0;
end = value + len;

#if (NGX_SUPPRESS_WARN)
day = 32;
year = 2038;
#endif

for (p = value; p < end; p++) {
if (*p == ',') {
break;
}

if (*p == ' ') {
fmt = isoc;
break;
}
}

for (p++; p < end; p++)
if (*p != ' ') {
break;
}

if (end - p < 18) {
return NGX_ERROR;
}

if (fmt != isoc) {
if (*p < '0' || *p > '9' || *(p + 1) < '0' || *(p + 1) > '9') {
return NGX_ERROR;
}

day = (*p - '0') * 10 + *(p + 1) - '0';
p += 2;

if (*p == ' ') {
if (end - p < 18) {
return NGX_ERROR;
}
fmt = rfc822;

} else if (*p == '-') {
fmt = rfc850;

} else {
return NGX_ERROR;
}

p++;
}

switch (*p) {

case 'J':
month = *(p + 1) == 'a' ? 0 : *(p + 2) == 'n' ? 5 : 6;
break;

case 'F':
month = 1;
break;

case 'M':
month = *(p + 2) == 'r' ? 2 : 4;
break;

case 'A':
month = *(p + 1) == 'p' ? 3 : 7;
break;

case 'S':
month = 8;
break;

case 'O':
month = 9;
break;

case 'N':
month = 10;
break;

case 'D':
month = 11;
break;

default:
return NGX_ERROR;
}

p += 3;

if ((fmt == rfc822 && *p != ' ') || (fmt == rfc850 && *p != '-')) {
return NGX_ERROR;
}

p++;

if (fmt == rfc822) {
if (*p < '0' || *p > '9' || *(p + 1) < '0' || *(p + 1) > '9'
|| *(p + 2) < '0' || *(p + 2) > '9'
|| *(p + 3) < '0' || *(p + 3) > '9')
{
return NGX_ERROR;
}

year = (*p - '0') * 1000 + (*(p + 1) - '0') * 100
+ (*(p + 2) - '0') * 10 + *(p + 3) - '0';
p += 4;

} else if (fmt == rfc850) {
if (*p < '0' || *p > '9' || *(p + 1) < '0' || *(p + 1) > '9') {
return NGX_ERROR;
}

year = (*p - '0') * 10 + *(p + 1) - '0';
year += (year < 70) ? 2000 : 1900;
p += 2;
}

if (fmt == isoc) {
if (*p == ' ') {
p++;
}

if (*p < '0' || *p > '9') {
return NGX_ERROR;
}

day = *p++ - '0';

if (*p != ' ') {
if (*p < '0' || *p > '9') {
return NGX_ERROR;
}

day = day * 10 + *p++ - '0';
}

if (end - p < 14) {
return NGX_ERROR;
}
}

if (*p++ != ' ') {
return NGX_ERROR;
}

if (*p < '0' || *p > '9' || *(p + 1) < '0' || *(p + 1) > '9') {
return NGX_ERROR;
}

hour = (*p - '0') * 10 + *(p + 1) - '0';
p += 2;

if (*p++ != ':') {
return NGX_ERROR;
}

if (*p < '0' || *p > '9' || *(p + 1) < '0' || *(p + 1) > '9') {
return NGX_ERROR;
}

min = (*p - '0') * 10 + *(p + 1) - '0';
p += 2;

if (*p++ != ':') {
return NGX_ERROR;
}

if (*p < '0' || *p > '9' || *(p + 1) < '0' || *(p + 1) > '9') {
return NGX_ERROR;
}

sec = (*p - '0') * 10 + *(p + 1) - '0';

if (fmt == isoc) {
p += 2;

if (*p++ != ' ') {
return NGX_ERROR;
}

if (*p < '0' || *p > '9' || *(p + 1) < '0' || *(p + 1) > '9'
|| *(p + 2) < '0' || *(p + 2) > '9'
|| *(p + 3) < '0' || *(p + 3) > '9')
{
return NGX_ERROR;
}

year = (*p - '0') * 1000 + (*(p + 1) - '0') * 100
+ (*(p + 2) - '0') * 10 + *(p + 3) - '0';
}

if (hour > 23 || min > 59 || sec > 59) {
return NGX_ERROR;
}

if (day == 29 && month == 1) {
if ((year & 3) || ((year % 100 == 0) && (year % 400) != 0)) {
return NGX_ERROR;
}

} else if (day > mday[month]) {
return NGX_ERROR;
}

/*
* shift new year to March 1 and start months from 1 (not 0),
* it is needed for Gauss' formula
*/

if (--month <= 0) {
month += 12;
year -= 1;
}

/* Gauss' formula for Gregorian days since March 1, 1 BC */

time = (uint64_t) (
/* days in years including leap years since March 1, 1 BC */

365 * year + year / 4 - year / 100 + year / 400

/* days before the month */

+ 367 * month / 12 - 30

/* days before the day */

+ day - 1

/*
* 719527 days were between March 1, 1 BC and March 1, 1970,
* 31 and 28 days were in January and February 1970
*/

- 719527 + 31 + 28) * 86400 + hour * 3600 + min * 60 + sec;

#if (NGX_TIME_T_SIZE <= 4)

if (time > 0x7fffffff) {
return NGX_ERROR;
}

#endif

return (time_t) time;
}

[qq_15455143]

这个算法是：把时间轴向前移2个月，使2月成为前一年最后一个月,从而屏蔽掉了闰年问题。367是每个月已过天数相加的和(设定2月为30天)，假定当前输入的日期是2月，经过前面的处理，年份变为year-1,月份变为12，367*month/12-30，2月这个设定的30天被减掉，后面的day-1是当前月份已经过去的天数(-1是因为当天还没过去)