dates.c

void secs_to_hms(long ts,long *h,long*mn,long*s){
    if(ts>0){
        *h=ts/3600;
        *mn=ts%3600/60;
        *s=ts%60;
    }else{
        *h=23+ts/3600;
        *mn=59+ts%3600/60;
        *s=59+ts%60;
    }
}


int monthdays[]={31,28,31,30,31,30,31,31,30,31,30,31,
                 31,29,31,30,31,30,31,31,30,31,30,31};

int y_is_leap(long y){
        return (y%4==0&&y%100!=0||y%400==0);
}

long y_to_days(long y){
    long d=0;
    if(y>2000){
        y=((y-2000)%400+2000);
        d+=(y-2000)/400*146097;
    }else{
        y=(2000-(2000-y)%400);
        d-=(2000-y)/400*146097;
    }
    while(y>2000){
        y--;
        d+=365+y_is_leap(y);
    }
    while(y<2000){
        d-=365+y_is_leap(y);
        y++;
    }
    return d;
}

long days_to_dofw(long d){
    if(d>=0){
        return (d+4)%7+1;
    }else{// day zero was a friday (5).
        return 7-(2-d)%7;
    }
}

long ymd_to_days(long y,long m,long d){
    while(m>12){y++;m-=12;}
    while(m<1){y--;m+=12;}
    while(m>1){
        int leap = y_is_leap(y);
        int md = monthdays[m-2+12*leap];
        d+=md;
        m--;
    }
    d += y_to_days(y);
    //400y = 365*400 + 100 - 4 + 1 =146097
    return d;
}

void days_to_ymd(long d,long *yp,long *mp,long *dp){
    long y=2000;
    long m=1;
    long yd=366;
    if(d>146097){
        y+=d/146097*400;
        d=d%146097;
    }
    if(d<-146097){
        y-=(-d)/146097*400;
        d= -((-d)%146097);
    }
    while(d>yd){
        d-=yd;
        y++;
        int leap = (y%4==0&&y%100!=0||y%400==0);
        yd=365+leap;
    }
    while(d<1){
        y--;
        int leap = (y%4==0&&y%100!=0||y%400==0);
        yd=365+leap;
        d+=yd;
    }
    long md = 31;
    while(d>md){
        d-=md;
        m++;

        int leap = (y%4==0&&!y%100==0||y%400==0);
        md = monthdays[m-1+12*leap];
    }
    *dp=d;
    *mp=m;
    *yp=y;
}