Out of bounds on the wrong variable with mktime

I am trying to add a user input date to an existing time struct but I am getting an error I do not understand. The complier is telling me the weekday is out of bounds when I am trying to modify the month date. Code is posted below.

struct tm date;
int m, d, y, n;
char buffer[80];
printf("Enter a date in mm/dd/yyyy format.\n");
scanf("%d/%d/%d", &m, &d, &y);

date.tm_mday = d;
date.tm_mon = m;
date.tm_year = y - 1900;
mktime(&date);

printf("How many days would you like to advance this time?");
scanf("%d", &n);
date.tm_mday += n;
mktime(&date);
strftime(buffer, sizeof(buffer), "%c", &date);
printf("Your new date is %c", buffer);

Solution

Problems

.tm_mon expects the months since January 0-11.

This usually means to subtract 1 from user's input. @dashboard
```
// date.tm_mon = m;
date.tm_mon = m - 1;
```

mktime(&date); reads all members aside from .tm_yday and .tm_wday and OP's only sets 3 of them. Best to initialize all members with {0} as we only known of 7 others and there might be more. @Jonathan Leffler

Code did not check the return value of mktime() for errors.

Use "%s" to print a string. This implies OP does not have a good compiler with warnings fully enable. Save time and enable all warnings with a good compiler.
```
char buffer[80];
printf("Your new date is %c", buffer);  // Bad specifier
```

Put this all together.

// struct tm date;
struct tm date = {0}; // Initialize all
int m, d, y, n;
char buffer[80];

printf("Enter a date in mm/dd/yyyy format.\n");
if (scanf("%d/%d/%d", &m, &d, &y) != 3) {
  Handle_error();  
}


date.tm_mday = d;
// date.tm_mon = m;
date.tm_mon = m - 1;
date.tm_year = y - 1900;
date.tm_isdst = -1; // Set dst flag to: let mktime figure it out
if (mktime(&date) == -1) {
  Handle_error();  
}

printf("How many days would you like to advance this time?");
scanf("%d", &n);
date.tm_mday += n;
if (mktime(&date) == -1) {
  Handle_error();  
}

strftime(buffer, sizeof(buffer), "%c", &date);
printf("Your new date is %s", buffer);