Stuck on this code. Can someone please help?

Question

Hello!
I was writing a code to convert US date(mm/dd/yyyy) to EU date and I got stuck.
the problem is that the loop:
while(date[i] != '\0'){
addchar(date[i],year);
i++;
}
changes the string date, although it's not supposed to.
Here is a part of my code. Could you take a look?
Thank you in advance ;)
-----------------------------------
//language: C

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

int index_of(char ch, char arr[]){
int i = 0;
while(i <= strlen(arr)){
if(arr[i] == ch) return i;
i++;
}
return -1;
}

int last_index_of(char ch, char arr[]){
long int i= strlen(arr)-1;
while(i >= 0){
if(arr[i] == ch) return i;
i--;
}
return -1;
}

void addchar(char ch,char str[]){//adds a character to the string
char arr[1];
arr[0]=ch;
strncat(str,arr,1);
}
//I've tested the functions above and they seem to be working correctly.

void convert(){
char date[100];//date = mm/dd/yyyy
scanf("%s", date);//enter a date
printf("date = %s\n", date); //erverything is fine
char year[1];
int i = last_index_of('/',date)+1;
printf("date = %s\n",date); //erverything is fine
while(date[i] != '\0'){
addchar(date[i],year);
i++;
}
printf("year = %s\n", year); //still everything is fine
printf("date = %s\n", date); //problem
}

int main() {
convert();
return 0;
}

Answer 1

Here is the problem:
char year[1];

It is writing past the end of year[] into the next variable on the stack, which is date[]. You need to declare at least 5 characters in year[] to store a 4-digit year plus null terminator. Also you should initialize year[0] = '\0'; to ensure strncat() starts in the right place.

Answer 2

Thanks!