Work time :)

input.txt

4
15.01. 17:00
16.01. 12:00
11.02. 14:00
30.01. 10:00

main.cpp

#include <fstream>
using namespace std;
ifstream in("input.txt");
ofstream out("output.txt");

struct P { int d, m, t;};

int s(const string &l, int s) { return stoi(l.substr(s, 2)); }

int main() {
    int c, i;
    string l;
    in >> c;
    P p[c];
    for (i = 0; i < c; ++i) {
        in >> l;
        p[i].d = s(l,0);
        p[i].m = s(l,3);
        in >> l;
        p[i].t = s(l,0) * 60 + s(l,3);
    }

    qsort(p, c, sizeof(struct P), [](const void *s1, const void *s2) {
        P *a = (P *) s1;
        P *b = (P *) s2;
        int r = a->m - b->m;
        if (r == 0) r = a->d - b->d;
        if (r == 0) r = a->t - b->t;
        return r;
    });

    int r = 0;
    for (i = 0; i < c; i+=2) {
        P s = p[i];
        P e = p[i+1];
        r += e.t - s.t + 1;
        r += (e.d - s.d) * 480;
        if (e.m != s.m) {
            for (int m = s.m; m < e.m; m++) {
                r += (m == 2 ? 28 : m % 2 == (m <= 7 ? 0 : 1) ? 30 : 31) * 480;
            }
        }
    }
    int h = r / 60;
    int m = r - (h * 60);
    out << h << (m > 9 ? ":" : ":0") << m;
}

main_readable.cpp

#include <fstream>
using namespace std;
ifstream in("input.txt");
ofstream out("output.txt");

struct Page { int day, month, minutes;};

int compare(const void *s1, const void *s2) {
    Page *a = (Page *) s1;
    Page *b = (Page *) s2;
    int r = a->month - b->month;
    if (r == 0) r = a->day - b->day;
    if (r == 0) r = a->minutes - b->minutes;
    return r;
}

int parseInt(const string &l, int pos) { return stoi(l.substr(pos, 2)); }

int main() {
    int c, i;
    string l;
    in >> c;
    Page pages[c];
    for (i = 0; i < c; ++i) {
        in >> l;
        pages[i].day = parseInt(l, 0);
        pages[i].month = parseInt(l, 3);
        in >> l;
        pages[i].minutes = parseInt(l, 0) * 60 + parseInt(l, 3);
    }

    qsort(pages, c, sizeof(struct Page), compare);

    int r = 0;
    for (i = 0; i < c; i+=2) {
        Page p1 = pages[i];
        Page p2 = pages[i + 1];
        r += p2.minutes - p1.minutes + 1;
        r += (p2.day - p1.day) * 480;
        if (p2.month != p1.month) {
            for (int m = p1.month; m < p2.month; m++) {
                r += (m == 2 ? 28 : m % 2 == (m <= 7 ? 0 : 1) ? 30 : 31) * 480;
            }
        }
    }
    int hours = r / 60;
    int minutes = r - (hours * 60);
    out << hours << (minutes > 9 ? ":" : ":0") << minutes;
}

output.txt

103:02