Writing a date checker in C
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.