So I was asked to write a code which would take in 2 dates of the format mm/dd/yyyy, validate that they were good dates (including leap years), then output the dates in chronological order. It works as intended for every date I throw at it but it seems way to complicated with all the nested if statement. Any suggestions on how to streamline it? Please keep in mind I'm a beginner and have only just learned to use arrays (pointers next week). Please give an example if you have a suggestion. "Use an array!" doesn't help so much. lol Thanks for your time.

/*
  Program assignment name: 2_Dates

  Author:Christopher D*****
*/

#include <stdio.h>

int main (void)
{ 
   int mm=0, dd=0, yyyy=0, mm2=0, dd2=0, yyyy2=0; 
   int first_date = 0, second_date = 0; 
   
   while(1) 
   {
      printf("Enter first date (mm/dd/yyyy): "); 
      scanf("%d /%d /%d", &mm, &dd, &yyyy);
      printf("\n");
      printf("Enter second date (mm/dd/yyyy): "); 
      scanf("%d /%d /%d", &mm2, &dd2, &yyyy2); 
      printf("\n");

      /* Checks to see if months are between 1-12, day 1-31, & years 0-9999 */
      if(yyyy>=1 && yyyy<=9999 && yyyy2>=1 && yyyy2<=9999 && 
         mm>=1 && mm<=12 && mm2>=1 && mm2<=12 && dd>=1 && dd<=31 && 
         dd2>=1 && dd2<=31)
      {
	 /* Checks months that don't have 31 days */
	 if(mm==2 && dd>28 && dd<30 || mm2==2 && dd2>28 && dd2<30)
	 {
	    /* Leap year test for February*/
	    if(((yyyy % 4 !=0 && yyyy % 100 ==0) || yyyy % 400 !=0) ||
	       ((yyyy2 % 4 !=0 && yyyy2 % 100 ==0) || yyyy2 % 400 !=0))
	    {
	       printf("February doesn't have that many days. ");
	       printf("Please try again.\n\n");
	       continue; /* Passes control to the end of the while loop */
	    }			
	 }
	 if(mm==4 && dd>30 || mm2==4 && dd>30)
	 {
	    printf("April doesn't have that many days. ");
	    printf("Please try again.\n\n");
	    continue;
	 }
	 if(mm==6 && dd>30 || mm2==6 && dd>30)
	 {
	    printf("June doesn't have that many days. ");
	    printf("Please try again.\n\n");
	    continue;
	 }
	 if(mm==9 && dd>30 || mm2==9 && dd>30)
	 {
	    printf("September doesn't have that many days. ");
	    printf("Please try again.\n\n");
	    continue;
	 }
	 if(mm==11 && dd>30 || mm2==11 && dd>30)
	 {
	    printf("November doesn't have that many days. ");
	    printf("Please try again.\n\n");
	    continue;
	 }
	 break; /* breaks from loop if input was a valid date */
      }
      
      printf("You have entered an invalid date. ");
      printf("Please try again.\n\n");
      
      while (getchar() != '\n'); /* Clears input buffer */
   }

   /* Converts dates into integer values */
   first_date = (yyyy * 10000) + (mm * 100) + dd; 
   second_date = (yyyy2 * 10000) + (mm2 * 100) + dd2;
   
   /* Compares these values and prints the earlier date first */
   if (first_date < second_date) 
   { 
      printf("%2d/%2d/%4d is earlier than %2d/%2d/%4d\n", 
	     mm, dd, yyyy, mm2, dd2, yyyy2);
   }
   else if (first_date > second_date)
   {
      printf("%2d/%2d/%4d is earlier than %2d/%2d/%4d\n", 
	     mm2, dd2, yyyy2, mm, dd, yyyy);
   }
   else if (first_date == second_date)
   {
      printf("The dates you entered are the same\n");
   }

   return 0;
}

The only real duplication you have is in printing the too many days error. Any other overcomplication comes from parsing two dates at the same time. I think a better option is a function that parses one date, then call it twice.

1) pass the dates to a function
2) evaluate failures first and call return false from the function early-on
--- this eliminates the use of "else" and can remove some of the nesting.

Agreed.
Make a function.
Test year -- return false if bad
Test month -- return false if bad
Test day <1 & >31 -- return false if bad
Make switch for:
31 day months
30 day months
Feb...

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