Writing a date checker in C

Question

I'm currently working on writing a date checker function in C and wondering how it can be improved upon.

Q: Write a function that is passed a month, day, and year and will determine if that date is valid. You can assume each parameter is passed in as an integer. Remember to check for leap year.

I currently have this and want to see if there are any beginner mistakes, if pointers could improve efficiency or redundancies in my code.

#include <stdio.h>
#include <stdbool.h>
bool is_valid_date(int month,int day ,int year) {
    //Validate year
    int days_on_month[]={31,28,31,30,31,30,31,31,30,31,30};
    if(year<0||year>2017) {
        //year not valid , stop validation , date is invalid
        return false;
    }
    bool isLeapYear;
    //Check for leap year , every 4 years there is a leap year
    if(year%4==0) {
        //if year is evenly divisble by 100 it must also be by 400
        if(year%100==0) {
            if(year%400==0) {
                isLeapYear=true;
            }else{
                isLeapYear=false;
            }
        }else{
            isLeapYear=true;
        }
    }
    //Validate months;
    if(month<1||month>12) {
        // not valid , stop validation , date is invalid
        return false;
    }
    //validate day
    if(month!=2) {
        //Valid Date
        if(day>=1&&day<=days_on_month[month-1]){return true;}
    }
    //February (changes no of days on leap year)
    else
    {
        if(isLeapYear) {
            if(day>=1&&day<=29){return true;}
        }else{
        if(day>=1&&day<=28){return false;}
        }
    }
    return false;
}

// test if function works
int main(void) {
    int day = 28;
    int month = 12;
    int year = 2012;
    printf("Is it true? Yes = 1 No = 0! = %d\n", is_valid_date(day, month, year));
    return 0;
}

Answer 1

Some quick formatting tips:

You use whitespace sparingly. In the example code, there is a lot of room for more whitespace that makes the code more readable. Something like:

if (year % 400 == 0) {
    isLeapYear = true;
}

It makes the code more easier to read.

Also, and this is not a recommendation but just an option you may want to consider, but you can move the if-else to a new line. I've always found that easier to read, and I've noticed that some (not all) teachers of mine tend to find that good formatting.

So for example:

if(year%400==0) {
    isLeapYear=true;
}else{
    isLeapYear=false;
}

This code can be rewritten as:

if(year%400==0) {
    isLeapYear=true;
}
else{
    isLeapYear=false;
}

**As an aside, upon receiving further advice from co-users on CodeReview in the comments, it is better to use curly braces, even if they are solely for one command in an if-statement, as it will help in detecting the scope of the if-statement with more ease.

Answer 2

You could format things nicely, or at least consistently.

Hardcoding in the current year is a bad idea.

if (year % 400 == 0) {
    isLeapYear = true;
} else if (year % 100 == 0) {
    isLeapYear = false;
} else if (year % 4 == 0) {
    isLeapYear = true;
}

You could put a day < 1 check once, before the other day checks.