void GregorianDay(struct rtc_time * tm)
{
int leapsToDate;
int
lastYear;
int day;
int MonthOffset[] = {
0,31,59,90,120,151,181,212,243,273,304,334 };
lastYear=tm->tm_year-1;
/*计算从公元元年到计数的前一年之中一共经历了多少个闰年*/
leapsToDate = lastYear/4 - lastYear/100 +
lastYear/400;
/*如若计数的这一年为闰年，且计数的月份在2月之后，则日数加1，否则不加1*/
if((tm->tm_year%4==0)
&&
((tm->tm_year%100!=0) || (tm->tm_year%400==0))
&&
(tm->tm_mon>2)) {
/*
* We are past Feb. 29 in
a leap year
*/
day=1;
} else {
day=0;
}
day += lastYear*365 + leapsToDate + MonthOffset[tm->tm_mon-1] + tm->tm_mday; /*计算从公元元年元旦到计数日期一共有多少天*/
tm->tm_wday=day%7;
}
原文：http://www.cnblogs.com/liufang/p/3729358.html