Algorithm to add or subtract days from a date?

Question

I\'m trying to write a Date class in an attempt to learn C++.

I\'m trying to find an algorithm to add or subtract days to a date, where Day starts from 1 and Month s

Answer 1

I know it is an old question asked almost a decade before. But a few days before I came across the same for an assignment, and here is the answer as in here

// C++ program to find date after adding 
// given number of days. 
#include 
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; 
}