C program days between two dates

Question

I have written a program that should find the days between two dates, but it has some hiccups. The logic makes perfect sense in my head when I read through it, so

Answer 1

There are multiple problems in your code snippet.. but I must say it is a very good attempt. There are many short cuts to what you're try to achieve.

I have written the following program which finds the number of days between two given dates. You may use this as a reference.

#include <stdio.h>
#include <stdlib.h>

char *month[13] = {"None", "Jan", "Feb", "Mar", 
                   "Apr", "May", "June", "July", 
                   "Aug", "Sept", "Oct", 
                   "Nov", "Dec"};

/*
daysPerMonth[0] = non leap year
daysPerMonth[1] = leap year
*/
int daysPerMonth[2][13] = {{-1, 31, 28, 31, 30, 31, 30, 31, 31, 30, 31, 30, 31},
                           {-1, 31, 29, 31, 30, 31, 30, 31, 31, 30, 31, 30, 31}};

typedef struct _d {
    int day;        /* 1 to 31 */
    int month;      /* 1 to 12 */
    int year;       /* any */
}dt;

void print_dt(dt d)
{
    printf("%d %s %d \n", d.day, month[d.month], d.year);
    return;
}

int leap(int year)
{
    return ((year % 4 == 0 && year % 100 != 0) || year % 400 ==0) ? 1 : 0;
}

int minus(dt d1, dt d2)
{
    int d1_l = leap(d1.year), d2_l = leap(d2.year);
    int y, m;
    int total_days = 0;

    for (y = d1.year; y >= d2.year ; y--) {
        if (y == d1.year) {
            for (m = d1.month ; m >= 1 ; m--) {
                if (m == d1.month)  total_days += d1.day;
                else                total_days += daysPerMonth[leap(y)][m];
                // printf("%d - %5s - %d - %d \n", y, month[m], daysPerMonth[leap(y)][m], total_days);
            }
        } else if (y == d2.year) {
            for (m = 12 ; m >= d2.month ; m--) {
                if (m == d2.month)  total_days += daysPerMonth[leap(y)][m] - d2.day;
                else                total_days += daysPerMonth[leap(y)][m];
                // printf("%d - %5s - %d - %d \n", y, month[m], daysPerMonth[leap(y)][m], total_days);
            }
        } else {
            for (m = 12 ; m >= 1 ; m--) {
                total_days += daysPerMonth[leap(y)][m];
                // printf("%d - %5s - %d - %d \n", y, month[m], daysPerMonth[leap(y)][m], total_days);
            }
        }

    }

    return total_days;
}

int main(void)
{
    /* 28 Oct 2018 */
    dt d2 = {28, 10, 2018};

    /* 30 June 2006 */
    dt d1 = {30, 6, 2006};

    int days; 

    int d1_pt = 0, d2_pt = 0;

    if (d1.year  > d2.year)     d1_pt += 100;
    else                        d2_pt += 100;
    if (d1.month > d2.month)    d1_pt += 10;
    else                        d2_pt += 10;
    if (d1.day   > d2.day)      d1_pt += 1;
    else                        d2_pt += 1;

    days = (d1_pt > d2_pt) ? minus(d1, d2) : minus(d2, d1);

    print_dt(d1);
    print_dt(d2);
    printf("number of days: %d \n", days);

    return 0;
}

The output is as follows:

$ gcc dates.c 
$ ./a.out 
30 June 2006 
28 Oct 2018 
number of days: 4503 
$ 

Note: this is not a complete program. It lacks input validation.

Hope it helps!

Answer 2

Reduce all month indexes by 1.

What I mean to say is January will correspond to daysPerMonth[0] or daysPerMonthLeap[0] and not daysPerMonth[1] or daysPerMonthLeap[1]. The reason for this being array indexes start from 0.

So, wherever you are using month1, month2 insidedaysPerMonth[] or daysPerMonthLeap[], use month1-1 and month2-1 instead.

I hope this is clear enough. Otherwise, feel free to comment.

Answer 3

//Difference/Duration between two dates
//No need to calculate leap year offset or anything
// Author: Vinay Kaple
# include <iostream>
using namespace std;
int main(int argc, char const *argv[])
{
    int days_add, days_sub, c_date, c_month, b_date, b_month, c_year, b_year;
    cout<<"Current Date(dd mm yyyy): ";
    cin>>c_date>>c_month>>c_year;
    cout<<"Birth Date(dd mm yyyy): ";
    cin>>b_date>>b_month>>b_year;
    int offset_month[12] = {0,31,59,90,120,151,181,212,243,273,304,334};
    days_add = c_date + offset_month[c_month-1];
    days_sub = b_date + offset_month[b_month-1];
    int total_days = (c_year-b_year)*365.2422 + days_add - days_sub+1;
    cout<<"Total days: "<<total_days<<"\n";
    int total_seconds = total_days*24*60*60;
    cout<<"Total seconds: "<<total_seconds<<"\n";
    return 0;
}

Answer 4

First, that leap function feels overly complicated; you don't need to do both dates in one function call, and I'm sure that can be written more succinctly so that it is more obviously correct. Here's a version I've got laying around that isn't succinct but I'm confident it is easy to check the logic:

int is_leap_year(int year) {
        if (year % 400 == 0) {
                return 1;
        } else if (year % 100 == 0) {
                return 0;
        } else if (year % 4 == 0) {
                return 1;
        } else {
                return 0;
        }
}

You could call it like this:

int year1, year2, leap1, leap2;
year1 = get_input();
year2 = get_input();
leap1 = is_leap_year(year1);
leap2 = is_leap_year(year2);

No pointers and significantly less code duplication. Yes, I know that is_leap_year() can be reduced to a single if(...) statement, but this is easy for me to read.

Second, I think you're got a mismatch between 0-indexed arrays and 1-indexed human months:

            if(*month1 < 1 || *month1 > 12)

vs

    int daysPerMonth[] = {31,28,31,30,31,30,31,31,30,31,30,31};

Third, I think that days per month can be calculated slightly nicer:

int days_in_month(int month, int year) {
        int leap = is_leap_year(year);
        /*               J   F   M   A   M   J   J   A   S   O   N   D */
        int days[2][12] = {{31, 28, 31, 30, 31, 30, 31, 31, 30, 31, 30, 31},
                           {31, 29, 31, 30, 31, 30, 31, 31, 30, 31, 30, 31}};
        if (month < 0 || month > 11 || year < 1753)
                return -1;

        return days[leap][month];
}

Here, I assume January is 0; you would need to force the rest of the code to match. (I learned this double-array trick from The Elements of Programming Style (page 54).) The best part of using a routine like this is that it removes the leap condition from the difference calculation.

Fourth, you're indexing arrays outside their bounds:

            for(i = month1 + 1; i <= 12; i++)
            {
                if(leap1 == 1)
                    total += daysPerMonthLeap[i];

This is just another instance of the problem with 0-indexed arrays and 1-indexed months -- but be sure that you fix this, too, when you fix the months.

I have a fear that I haven't yet found all the issues -- you may find it easier to sort the first and the second date after input and remove all that validation code -- and then use names before and after or something to give names that are easier to think through in the complicated core of the calculation.

Answer 5

This is not a complete answer. I just wanted to mention a better way to calculate leap year (this is taken from The C Programming Language - Page #41)

if ((year % 4 == 0 && year % 100 != 0) || year % 400 ==0)
    printf("%d is a leap year \n", year);
else
    printf("%d is not a leap year \n", year);

Answer 6

Change

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

to

int daysPerMonth[] = {0,31,28,31,30,31,30,31,31,30,31,30,31};
int daysPerMonthLeap[] = {0,31,29,31,30,31,30,31,31,30,31,30,31};

i.e. pad the arrays at the beginning since all the code relies on the array values to start at element 1 rather than element 0.

That will get rid of the error you complained of.

The other problem is an off-by-one error when you add day2 to the total. In both cases you should add day2 - 1 rather than day2. This is also due to the date indexes starting at 1 instead of 0.

After I made these changes (plus a couple just to get the code to compile), it works properly.