A C program to calculate age from birth date.

age.c

// prints age given date of birth in form dd/mm/yyyy
// compile with: gcc age.c -o age
// Author: Dr. Abiira Nathan
// Date: May, 2022
// Version: 1.0
// License: MIT License
// Copyright (c) 2020 Dr. Abiira Nathan

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

typedef struct
{
  int date, month, year;
} date;

// returns the number of digits in an integer n
int num_digits(int n)
{
  int count = 0;
  while (n > 0)
  {
    n /= 10;
    count++;
  }
  return count;
}

// Check if year is leap year
bool is_leap(int year)
{
  if (year % 4 == 0 && year % 100 != 0 || year % 400 == 0)
    return true;
  else
    return false;
}

/*
Returns the age in a human readable format.
  If age is greater than or equal to 1 year, it will return the age in years +/- months(if years < 2)
  If the age is less than 1 year, it will return the age in months +/- days(months < 2).
  If the age is less than 1 month, it will return the age in days.
*/
char *humanize(int years, int months, int days)
{
  // return date of birth as a string
  // e.g "22 years" or "4 years, 3 months" or "5 days"

  char *str = (char *)malloc(sizeof(char) * 11);

  // years or years and months
  if (years > 0)
  {
    sprintf(str, "%d year%s", years, (years > 1) ? "s" : "");

    if (months > 0 && years < 2)
      sprintf(str, "%s, %d month%s", str, months, (months > 1) ? "s" : "");

    return str;
  }

  // months or months and days
  if (months > 0)
  {
    sprintf(str, "%d month%s", months, (months > 1) ? "s" : "");

    if (days > 0 && months < 2)
      sprintf(str, "%s, %d day%s", str, days, (days > 1) ? "s" : "");

    return str;
  }

  // days
  sprintf(str, "%d day%s", days, (days > 1) ? "s" : "");
  return str;
}

// calculates age given date of birth in days, months, years
// returns the date struct with the age in years, months, days
date findAge(int birth_date, int birth_month, int birth_year)
{
  struct tm *now_t;

  time_t current = time(NULL);
  now_t = localtime(&current);

  int current_date, current_month, current_year;
  current_date = now_t->tm_mday;
  current_month = now_t->tm_mon + 1;
  current_year = now_t->tm_year + 1900;

  // days of every month
  int month[] = {31, 28, 31, 30, 31, 30, 31, 31, 30, 31, 30, 31};

  // if leap year
  if (is_leap(current_year))
  {
    month[1] = 29;
  }

  // if birth date is greater then current birth
  // month then do not count this month and add 30
  // to the date so as to subtract the date and
  // get the remaining days
  if (birth_date > current_date)
  {
    current_date = current_date + month[birth_month - 1];
    current_month = current_month - 1;
  }

  // if birth month exceeds current month, then do
  // not count this year and add 12 to the month so
  // that we can subtract and find out the difference
  if (birth_month > current_month)
  {
    current_year = current_year - 1;
    current_month = current_month + 12;
  }

  // calculate date, month, year
  date d;
  d.year = current_year - birth_year;
  d.month = current_month - birth_month;
  d.date = current_date - birth_date;
  return d;
}

int main(void)
{

  date dob;
  struct tm bd_t;

  printf("Enter the date of birth(dd/mm/yyyy): ");
  int res = scanf("%d/%d/%d", &dob.date, &dob.month, &dob.year);
  if (res != 3 || dob.date < 1 || dob.date > 31 ||
      dob.month < 1 || dob.month > 12 || dob.year < 1 || num_digits(dob.year) != 4)
  {

    printf("Invalid birth date: %.2d/%.2d/%4d\n", dob.date, dob.month, dob.year);
    return EXIT_FAILURE;
  }

  date age = findAge(dob.date, dob.month, dob.year);
  char *age_str = humanize(age.year, age.month, age.date);
  printf("Age: %s\n", age_str);
  return EXIT_SUCCESS;
}