#include <ctime>

// Adjust date by a number of days +/-
void DatePlusDays( struct tm* date, int days )
{
    const time_t ONE_DAY = 24 * 60 * 60 ;

    // Seconds since start of epoch
    time_t date_seconds = mktime( date ) + (days * ONE_DAY) ;

    // Update caller's date
    // Use localtime because mktime converts to UTC so may change date
    *date = *localtime( &date_seconds ) ; ;
}

#include <iostream>

int main()
{
    struct tm date = { 0, 0, 12 } ;  // nominal time midday (arbitrary).
    int year = 2010 ;
    int month = 2 ;  // February
    int day = 26 ;   // 26th

    // Set up the date structure
    date.tm_year = year - 1900 ;
    date.tm_mon = month - 1 ;  // note: zero indexed
    date.tm_mday = day ;       // note: not zero indexed

    // Date, less 100 days
    DatePlusDays( &date, -100 ) ;

    // Show time/date using default formatting
    std::cout << asctime( &date ) << std::endl ;
}

m = 1;
while(d > numberOfDaysInMonth(m, y)) {
    d -= numberOfDaysInMonth(m, y);
    m++;
}
return date with year = y, month = m, day = d;

while(d >= 365) {
    d -= 365;
    if(isLeapYear(y)) {
        d -= 1;
    }
    y++;
}
// now use the case where d is less than 365

tm* dateTimeAdd(const tm* const dateTime, const int& days, const int& hours, const int& mins, const int& secs) {
    tm* newTime = new tm;
    memcpy(newTime, dateTime, sizeof(tm));

    newTime->tm_mday += days;
    newTime->tm_hour += hours;
    newTime->tm_min += mins;
    newTime->tm_sec += secs;

    time_t nt_seconds = mktime(newTime) - timezone;
    delete newTime;

    return gmtime(&nt_seconds);
}

time_t t = time(NULL);
tm* utc = gmtime(&t);
tm* newUtc = dateTimeAdd(utc, -5, 0, 0, 0); //subtract 5 days

#include <iostream>
#include <string>

using namespace std;

class Date {
public:
    Date(size_t year, size_t month, size_t day):m_year(year), m_month(month), m_day(day) {}
    ~Date() {}

    // Add specified number of days to date
    Date operator + (size_t days) const;

    // Subtract specified number of days from date
    Date operator - (size_t days) const;

    size_t Year()  { return m_year; }
    size_t Month() { return m_month; }
    size_t Day()   { return m_day; }

    string DateStr();
private:
    // Leap year check
    inline bool LeapYear(int year) const
        { return year % 4 == 0 && (year % 100 != 0 || year % 400 == 0); }

    // Holds all max days in a general year
    static const int MaxDayInMonth[13];

    // Private members
    size_t m_year;
    size_t m_month;
    size_t m_day;
};

// Define MaxDayInMonth
const int Date::MaxDayInMonth[13] = {0, 31, 28, 31, 30, 31, 30, 31, 31, 30, 31, 30, 31};

//===========================================================================================
/// Add specified number of days to date
Date Date::operator + (size_t days) const {
    // Maximum days in the month
    int nMaxDays(MaxDayInMonth[m_month] + (m_month == 2 && LeapYear(m_year) ? 1 : 0));

    // Initialize the Year, Month, Days
    int nYear(m_year);
    int nMonth(m_month);
    int nDays(m_day + days);

    // Iterate till it becomes a valid day of a month
    while (nDays > nMaxDays) {
        // Subtract the max number of days of current month
        nDays -= nMaxDays;

        // Advance to next month
        ++nMonth;

        // Falls on to next year?
        if (nMonth > 12) {
            nMonth = 1; // January
            ++nYear;    // Next year
        }

        // Update the max days of the new month
        nMaxDays = MaxDayInMonth[nMonth] + (nMonth == 2 && LeapYear(nYear) ? 1 : 0);
    }

    // Construct date
    return Date(nYear, nMonth, nDays);
}

//===========================================================================================
/// Subtract specified number of days from date
Date Date::operator - (size_t days) const {
    // Falls within the same month?
    if (0 < (m_day - days)) {
        return Date(m_year, m_month, m_day - days);
    }

    // Start from this year
    int nYear(m_year);

    // Start from specified days and go back to first day of this month
    int nDays(days);
    nDays -= m_day;

    // Start from previous month and check if it falls on to previous year
    int nMonth(m_month - 1);
    if (nMonth < 1) {
        nMonth = 12; // December
        --nYear;     // Previous year
    }

    // Maximum days in the current month
    int nDaysInMonth = MaxDayInMonth[nMonth] + (nMonth == 2 && LeapYear(nYear) ? 1 : 0);

    // Iterate till it becomes a valid day of a month
    while (nDays >= 0) {
        // Subtract the max number of days of current month
        nDays -= nDaysInMonth;

        // Falls on to previous month?
        if (nDays > 0) {
            // Go to previous month
            --nMonth;

            // Falls on to previous year?
            if (nMonth < 1) {
                nMonth = 12; // December
                --nYear;     // Previous year
            }
        }

        // Update the max days of the new month
        nDaysInMonth = MaxDayInMonth[nMonth] + (nMonth == 2 && LeapYear(nYear) ? 1 : 0);
    }

    // Construct date
    return Date(nYear, nMonth, (0 < nDays ? nDays : -nDays));
}

//===========================================================================================
/// Get the date string in yyyy/mm/dd format
string Date::DateStr() {
    return to_string(m_year)
        + string("/")
        + string(m_month < 10 ? string("0") + to_string(m_month) : to_string(m_month))
        + string("/")
        + string(m_day < 10 ? string("0") + to_string(m_day) : to_string(m_day));
}


int main() {
    // Add n days to a date
    cout << Date(2017, 6, 25).DateStr() << " + 10 days = "
         << (Date(2017, 6, 25) /* Given Date */ + 10 /* Days to add */).DateStr() << endl;

    // Subtract n days from a date
    cout << Date(2017, 6, 25).DateStr() << " - 10 days = "
         << (Date(2017, 6, 25) /* Given Date */ - 10 /* Days to subract */).DateStr() << endl;

    return 0;
}

Output
2017/06/25 + 10 days = 2017/07/05
2017/06/25 - 10 days = 2017/06/15

// C++ program to find date after adding
// given number of days.
#include<bits/stdc++.h>
using namespace std;

// Return if year is leap year or not.
bool isLeap(int y)
{
    if (y%100 != 0 && y%4 == 0 || y %400 == 0)
        return true;

    return false;
}

// Given a date, returns number of days elapsed
// from the beginning of the current year (1st
// jan).
int offsetDays(int d, int m, int y)
{
    int offset = d;

    switch (m - 1)
    {
    case 11:
        offset += 30;
    case 10:
        offset += 31;
    case 9:
        offset += 30;
    case 8:
        offset += 31;
    case 7:
        offset += 31;
    case 6:
        offset += 30;
    case 5:
        offset += 31;
    case 4:
        offset += 30;
    case 3:
        offset += 31;
    case 2:
        offset += 28;
    case 1:
        offset += 31;
    }

    if (isLeap(y) && m > 2)
        offset += 1;

    return offset;
}

// Given a year and days elapsed in it, finds
// date by storing results in d and m.
void revoffsetDays(int offset, int y, int *d, int *m)
{
    int month[13] = { 0, 31, 28, 31, 30, 31, 30,
                    31, 31, 30, 31, 30, 31 };

    if (isLeap(y))
        month[2] = 29;

    int i;
    for (i = 1; i <= 12; i++)
    {
        if (offset <= month[i])
            break;
        offset = offset - month[i];
    }

    *d = offset;
    *m = i;
}

// Add x days to the given date.
void addDays(int d1, int m1, int y1, int x)
{
    int offset1 = offsetDays(d1, m1, y1);
    int remDays = isLeap(y1)?(366-offset1):(365-offset1);

    // y2 is going to store result year and
    // offset2 is going to store offset days
    // in result year.
    int y2, offset2;
    if (x <= remDays)
    {
        y2 = y1;
        offset2 = offset1 + x;
    }

    else
    {
        // x may store thousands of days.
        // We find correct year and offset
        // in the year.
        x -= remDays;
        y2 = y1 + 1;
        int y2days = isLeap(y2)?366:365;
        while (x >= y2days)
        {
            x -= y2days;
            y2++;
            y2days = isLeap(y2)?366:365;
        }
        offset2 = x;
    }

    // Find values of day and month from
    // offset of result year.
    int m2, d2;
    revoffsetDays(offset2, y2, &d2, &m2);

    cout << "d2 = " << d2 << ", m2 = " << m2
        << ", y2 = " << y2;
}

// Driven Program
int main()
{
    int d = 14, m = 3, y = 2015;
    int x = 366;

    addDays(d, m, y, x);

    return 0;
}