Month Calculator
Operation Type
Start Date
→
End Date
Start Date
Months to Add
0 months
(0 years and 0 months)
Enter dates to calculate the months between them.
How to Use the Months Calculator
1. Choose your calculation type: Add Months, Subtract Months, or find the Months Between two dates.
2. Enter the required dates using the date pickers. For adding or subtracting, also enter the number of months.
3. The result is calculated instantly. For "Months Between", you can check "Include End Day" for a more inclusive count.
The Logic to Calculate Months Between Dates
// Core logic to find the difference in months
let yearDiff = endDate.getFullYear() - startDate.getFullYear();
let monthDiff = endDate.getMonth() - startDate.getMonth();
let dayDiff = endDate.getDate() - startDate.getDate();
let totalMonths = (yearDiff * 12) + monthDiff;
// Adjust if the end day is earlier in the month
if (dayDiff < 0) {
totalMonths--;
}
Practical Applications
Financial Planning
Calculate loan terms, investment durations, or subscription periods in months.
Project Management
Determine the total duration of a project in months for accurate timeline reporting.
Personal Milestones
Find the number of months until a birthday, anniversary, or other important event.
What is a "Month"?
A "month" is a tricky unit for calculation because its length varies (28, 29, 30, or 31 days). This calculator defines "adding one month" as moving to the same day number in the next month. This is the most common and intuitive way to handle date math involving months.
Handling End-of-Month Dates
Calculation Gotcha!
What happens when you add 1 month to January 31? Since February doesn't have 31 days, our calculator correctly adjusts the result to the last day of February (the 28th or 29th). It's designed to never produce an invalid date like "February 31."
Months vs. Other Time Units
| Unit | Best Use Case | Example |
|---|---|---|
| Months | For billing cycles, subscriptions, and calendar-based events. | "3 months from Jan 15" is Apr 15. |
| Weeks | For short-term project sprints and recurring weekly events. | "4 weeks from Jan 15" is Feb 12. |
| Days | For precise, short-term deadlines and countdowns. | "30 days from Jan 15" is Feb 14. |
Fun Facts About Months
The word "month" is related to the word "Moon." Early calendars were based on the cycles of the moon.
July and August are both 31 days long because they were named after powerful Roman leaders, Julius Caesar and Augustus, who both wanted their month to be long.
February originally had 30 days but was shortened over time as other months were extended.
Frequently Asked Questions
Why does the result change when I check "Include End Day"? ›
By default, we count "full months" passed. For example, from Jan 15 to Feb 14 is not a full month. Checking the box makes the calculation inclusive, so Jan 15 to Feb 15 would count as exactly 1 month.
How does this calculator differ from a days calculator?›
This tool operates on the concept of calendar months. Calculating "30 days from now" will give a different result than "1 month from now" unless the current month has exactly 30 days.