`datediff` is a simple C program that calculates the number of days between two dates provided in the format `YYYY-MM-DD`. It is a lightweight and efficient utility for quick date difference calculations

datediff.c

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

// Function to convert a date string to a struct tm
int parse_date(const char *date_str, struct tm *date)
{
  memset(date, 0, sizeof(struct tm));

  if (sscanf(date_str, "%4d-%2d-%2d", &date->tm_year, &date->tm_mon,
             &date->tm_mday) != 3) {
    return -1;  // Error parsing date
  }

  date->tm_year -= 1900;  // tm_year is years since 1900
  date->tm_mon -= 1;      // tm_mon is zero-based

  return 0;
}

int main(int argc, char *argv[])
{
  if (argc != 3) {
    printf("Usage: %s <date1: YYYY-MM-DD> <date2: YYYY-MM-DD>\n", argv[0]);
    return 1;
  }

  struct tm date1, date2;
  time_t time1, time2;

  // Parse the input dates
  if (parse_date(argv[1], &date1) != 0 || parse_date(argv[2], &date2) != 0) {
    printf("Invalid date format. Please use YYYY-MM-DD.\n");
    return 1;
  }

  // Convert struct tm to time_t
  time1 = mktime(&date1);
  time2 = mktime(&date2);

  if (time1 == -1 || time2 == -1) {
    printf("Error converting dates to time.\n");
    return 1;
  }

  // Calculate the difference in days
  double day_gap = difftime(time2, time1) / (60 * 60 * 24);
  printf("%.0f\n", day_gap);

  return 0;
}