title: C program days between two dates
body: <p>I have written a program that <em>should</em> find the days between two dates, but it has some hiccups.  The logic makes perfect sense in my head when I read through it, so I'm assuming I have some syntax errors that I keep glancing over or something.</p>
  
  <p>Firstly, when entering two dates in different years, the output is always off by about one month (31 in most cases, but 32 in one case...go figure).  Second, two dates exactly one month apart will return the number of days in the second month (i.e. 1/1/1 to 2/1/1 yields 28).  There are inevitably some other weird things that this program does, but I am hoping that is enough information to help you guys figure out what I'm doing wrong.  For the life of me I can't figure this one out on my own.  I am relatively new to C, so please be gentle  =)</p>
  
  <p>Thanks</p>
  
  <pre><code>// Calculates the number of calendar days between any two dates in history (beginning with 1/1/1).
  
  #include &lt;stdio.h&gt;
  #include &lt;stdlib.h&gt;
  
  void leap(int year1, int year2, int *leap1, int *leap2);
  void date(int *month1, int *day1, int *year1, int *month2, int *day2, int *year2, int *leap1, int *leap2);
  
  int main(void)
  {
          int month1, day1, year1, month2, day2, year2, leap1, leap2;
          int daysPerMonth[] = {31,28,31,30,31,30,31,31,30,31,30,31};
          int daysPerMonthLeap[] = {31,29,31,30,31,30,31,31,30,31,30,31};
  
          leap(year1, year2, &amp;leap1, &amp;leap2);
          date(&amp;month1, &amp;day1, &amp;year1, &amp;month2, &amp;day2, &amp;year2, &amp;leap1, &amp;leap2);
  
          if(year1 == year2)
          {
                  int i, total;
  
                  if(month1 == month2)                            // Total days if month1 == month2
                  {
                          total = day2 - day1;
                          printf("There are %d days between the two dates.", total);
                  }
                  else
                  {
                      if(leap1 == 1)
                          total = daysPerMonthLeap[month1] - day1;
                      else
                          total = daysPerMonth[month1] - day1;
  
                      for(i = month1 + 1; i &lt; month2; i++)        // Days remaining between dates (excluding last month)
                      {
                          if(leap1 == 1)
                              total += daysPerMonthLeap[i];
                          else
                              total += daysPerMonth[i];
                      }
  
                      total += day2;                              // Final sum of days between dates (including last month)
  
                      printf("There are %d days between the two dates.", total);
                  }
          }
          else                                                    // If year1 != year2 ...
          {
                  int i, total, century1 = ((year1 / 100) + 1) * 100, falseleap = 0;
  
                  if(leap1 == 1)
                      total = daysPerMonthLeap[month1] - day1;
                  else
                      total = daysPerMonth[month1] - day1;
  
                  for(i = month1 + 1; i &lt;= 12; i++)               // Day remaining in first year
                  {
                      if(leap1 == 1)
                          total += daysPerMonthLeap[i];
                      else
                          total += daysPerMonth[i];
                  }
  
                  for(i = 1; i &lt; month2; i++)                     // Days remaining in final year (excluding last month)
                  {
                      if(leap2 == 1)
                          total += daysPerMonthLeap[i];
                      else
                          total += daysPerMonth[i];
                  }
  
                  int leapcount1 = year1 / 4;                     // Leap years prior to and including first year
                  int leapcount2 = year2 / 4;                     // Leap years prior to and NOT including final year
                  if(year2 % 4 == 0)
                          leapcount2 -= 1;
  
                  int leaptotal = leapcount2 - leapcount1;        // Leap years between dates
  
                  for(i = century1; i &lt; year2; i += 100)          // "False" leap years (divisible by 100 but not 400)
                  {
                          if((i % 400) != 0)
                                  falseleap += 1;
                  }
  
                  total += 365 * (year2 - year1 - 1) + day2 + leaptotal - falseleap;      // Final calculation
                  printf("There are %d days between the two dates.", total);
          }
          return 0;
  }
  
  void leap(int year1, int year2, int *leap1, int *leap2)             // Determines if first and final years are leap years
  {
          if(year1 % 4 == 0)
          {
                  if(year1 % 100 == 0)
                  {
                          if(year1 % 400 == 0)
                                  *leap1 = 1;
                          else
                                  *leap1 = 0;
                  }
                  else
                          *leap1 = 1;
          }
          else
                  *leap1 = 0;
  
          if(year2 % 4 == 0)
          {
                  if(year2 % 100 == 0)
                  {
                          if(year2 % 400 == 0)
                                  *leap2 = 1;
                          else
                                  *leap2 = 0;
                                  }
                  else
                          *leap2 = 1;
          }
          else
                  *leap2 = 0;
  }
  
  void date(int *month1, int *day1, int *year1, int *month2, int *day2, int *year2, int *leap1, int *leap2)
  {
          for(;;)                     // Infinite loop (exited upon valid input)
          {
                  int fail = 0;
                  printf("\nEnter first date: ");
                  scanf("%d/%d/%d", month1, day1, year1);
                  if(*month1 &lt; 1 || *month1 &gt; 12)
                  {
                          printf("Invalid entry for month.\n");
                          fail += 1;
                  }
                  if(*day1 &lt; 1 || *day1 &gt; 31)
                  {
                          printf("Invalid entry for day.\n");
                          fail += 1;
                  }
                  if(*year1 &lt; 1)
                  {
                          printf("Invalid entry for year.\n");
                          fail += 1;
                  }
                  if(daysPerMonth[month1] == 30 &amp;&amp; *day1 &gt; 30)
                  {
                          printf("Invalid month and day combination.\n");
                          fail += 1;
                  }
                  if(*month1 == 2)
                  {
                          if(*leap1 == 1 &amp;&amp; *day1 &gt; 29)
                          {
                              printf("Invalid month and day combination.\n");
                              fail += 1;
                          }
                          else if(*day1 &gt; 28)
                          {
                              printf("Invalid month and day combination.\n");
                              fail += 1;
                          }
                  }
                  if(fail &gt; 0)
                          continue;
                  else
                          break;
          }
  
          for(;;)
          {
                  int fail = 0;
                  printf("\nEnter second date: ");
                  scanf("%d/%d/%d", month2, day2, year2);
                  if(*year1 == *year2)
                  {
                          if(*month1 &gt; *month2)
                          {
                                  printf("Invalid entry.\n");
                                  fail += 1;
                          }
                          if(*month1 == *month2 &amp;&amp; *day1 &gt; *day2)
                          {
                                  printf("Invalid entry.\n");
                                  fail += 1;
                          }
                  }
                  if(*month2 &lt; 1 || *month2 &gt; 12)
                  {
                          printf("Invalid entry for month.\n");
                          fail += 1;
                  }
                  if(*day2 &lt; 1 || *day2 &gt; 31)
                  {
                          printf("Invalid entry for day.\n");
                          fail += 1;
                  }
                  if(*year2 &lt; 1)
                  {
                          printf("Invalid entry for year.\n");
                          fail += 1;
                  }
                  if(daysPerMonth[month2] == 30 &amp;&amp; *day2 &gt; 30)
                  {
                          printf("Invalid month and day combination.\n");
                          fail += 1;
                  }
                  if(*month2 == 2)
                  {
                          if(*leap2 == 1 &amp;&amp; *day2 &gt; 29)
                          {
                              printf("Invalid month and day combination.\n");
                              fail += 1;
                          }
                          else if(*day2 &gt; 28)
                          {
                              printf("Invalid month and day combination.\n");
                              fail += 1;
                          }
                  }
                  if(fail &gt; 0)
                          continue;
                  else
                          break;
          }
  }
  </code></pre>