Codeeval Challenge Delta Time

delta_time.c

/* https://www.codeeval.com/open_challenges/166/ */
#include <stdio.h>
#include <stdlib.h>
#include <string.h>
#define MAX 1024

int time_difference(char* time1, char* time2);
void seconds_to_time(int seconds); 

int main(int argc, char** argv)
{
	if (argc < 2)
	{
		printf("You need to provide an input file");
		return 1;
	}

	FILE *f = fopen(argv[1], "r");


	char *line = malloc(MAX * sizeof(char));
	
	while (fgets(line, MAX, f)){
		
		if (line[0] == '\n'){ //skip empty lines
			continue;
		}
		
		char* first_part = strtok(line, " ");
		char* second_part = strtok(NULL, " ");
		
		int tdiff = time_difference(first_part, second_part); // This is where the issue lies!

		seconds_to_time(tdiff);

	}
	free(line);
	return 0;
}

int time_difference(char* time1, char* time2)
{
		
	char str[9];
	char str1[9];
	strncpy(str, time1, 9);
	strncpy(str1, time2, 9);
		
	int t1 = (atoi(strtok(str, ":")) * 3600) + (atoi(strtok(NULL, ":")) * 60) + atoi(strtok(NULL, ":"));
	
	int t2 = (atoi(strtok(str1, ":")) * 3600) + (atoi(strtok(NULL, ":")) * 60) + atoi(strtok(NULL, ":"));
	
	return abs(t1 - t2);
}

void seconds_to_time(int seconds) // will receive input from time_difference
{
	int hours = seconds / 3600;
	int minutes = seconds % 3600 / 60;
	int secs = seconds - (hours * 3600) - (minutes * 60);

	printf("%02d:%02d:%02d\n", hours, minutes, secs);
 
}