Difference in Months between Two Dates in JavaScript

Handling complexities - when it isn't as easy as eating sushi

There are scenarios where you need more than just a basic calculation. Partial months, negative differences, and varying month lengths are potential chopsticks in the works.

Rounding off month fractions - off the cuff

Partial months might need to be considered if dates are in the middle of the month. A few days more and you could be looking at an unfortunate month overhead. Your call, really.

Negative differences - time travellers' problems

When your start date (d1) is ahead of your end date (d2), you're essentially travelling back in time. Now, don't we all wish we could do that on Monday mornings? Anyway, a negative value is returned in such cases.

Accounting for month lengths - the 28, 30, 31 conundrum

February is the weird cousin who sometimes has 28 days and sometimes 29. Moreover, not all months have the same number of days, so there might be some issues if you're looking for exact precision.

To work around these, we expand our function:

const monthDiff = (d1, d2, roundUpFractionalMonths = false) => {
    let months = (d2.getFullYear() - d1.getFullYear()) * 12 + d2.getMonth() - d1.getMonth();
    // Subtract birthdays that haven't happened yet.
    const partialMonth = (d2.getDate() < d1.getDate());

    // They said, "age is just a number", but didn't say it could be fractional.
    if (roundUpFractionalMonths && partialMonth) {
        months++;
    }
    return months;
};

// Using this for fractional months is like adding an extra sushi roll, just because you can! 🍣🍣
console.log(monthDiff(new Date('2023-01-15'), new Date('2023-04-14'), true)); // 3


The new parameter roundUpFractionalMonths, when set to true, rounds up the month count if there's a partial month difference.

Inclusive and Exclusive - the guest list for your month party

You might need to include or exclude the end month in your calculation, much like planning a party and deciding who gets to attend.

Excluding the party poopers

By default, the function excludes the end month if its day component is earlier than that of the start date.

Including everyone

To make sure everyone gets an invite, you can adjust the function or just add 1 to your result, like sprinkling extra Tobiko on your sushi. Red but delicious! 🍣✨

Advanced methods - for the adventurous coders

For complex date-difference calculations or to keep things simple and less hairy (like a shaved cat 🐱‍👤), you can use:

Handy dandy libraries

Solutions such as date-fns or Day.js got you covered with functions like differenceInMonths.

Test your strength

By testing leap years, leap seconds, and timezone changes, you can ensure your date calculations are more robust than a sumo wrestler.

Uniform Date Handling

To avoid any formatting confusion, always use ISO 8601 formatted strings (YYYY-MM-DD). Just like wearing a uniform.