Click here to download datecalc.c.

/*********************************************
Days Past

This program reads a start date and an end date
from the user, and prints out the number of
days elapsed between the two.

The key to the problem is to write a function
"monthdays()" that takes a month name
and returns the number of days from New Year's
to the beginning of that month.
 *********************************************/
#include <stdio.h>
#include <string.h>
#include <stdlib.h>

typedef char cstring[128];

int monthdays(cstring mname);

/*********************************************
 ** main() function
 *********************************************/
int main() {
  int dayofmonth;
  cstring monthname;
  int daystart;
  int dayfinish;
  // Get # of days from New Year's to start
  printf("Enter start  date (e.g. 27 Mar): ");
  fflush(stdout);
  scanf(" %d %s", &dayofmonth, monthname);
  daystart = dayofmonth + monthdays(monthname);

  // Get # of days from New Year's to finish
  printf("Enter ending date (e.g. 13 Jul): ");
  fflush(stdout);
  scanf(" %d %s", &dayofmonth, monthname);
  dayfinish = dayofmonth + monthdays(monthname);

  // Print out number of days in between
  printf("That lasted %i days.\n", dayfinish - daystart + 1);

  return 0;
}

/*********************************************
 ** Reads a month and returns the number of
 ** days from New Year's to that month
 *********************************************/
int monthdays(cstring mname) {
  // Get # of days from month
  int days = 0;

  if (strcmp(mname, "Jan") == 0) { return days; }
  days += 31;

  if (strcmp(mname, "Feb") == 0) { return days; }
  days += 28;

  if (strcmp(mname, "Mar") == 0) { return days; }
  days += 31;

  if (strcmp(mname, "Apr") == 0) { return days; }
  days += 30;

  if (strcmp(mname, "May") == 0) { return days; }
  days += 31;

  if (strcmp(mname, "Jun") == 0) { return days; }
  days += 30;

  if (strcmp(mname, "Jul") == 0) { return days; }
  days += 31;

  if (strcmp(mname, "Aug") == 0) { return days; }
  days += 31;

  if (strcmp(mname, "Sep") == 0) { return days; }
  days += 30;

  if (strcmp(mname, "Oct") == 0) { return days; }
  days += 31;

  if (strcmp(mname, "Nov") == 0) { return days; }
  days += 30;

  if (strcmp(mname, "Dec") == 0) { return days; }
  days += 31;

  printf("ERROR: invalid month name.");
  exit(1);
}