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Get First Day of Month Between Two Dates?

Learn how to list the first day of every month between two dates in JavaScript, including the start and end month.

byDev Solutions

February 12, 2026

JavaScript function to get first day of each month between two dates with calendar icons and floating date cards

📆 JavaScript can figure out the first day of the month between two dates using its built-in tools.
📚 Libraries like date-fns, Luxon, and Moment.js give good choices. They help with parsing and timezone support more.
⚠️ Timezone problems can cause bugs when you work with local and UTC dates.
🚀 Generators save memory and work well when getting many month ranges in JavaScript.
📊 This method is important for programs like invoicing, analytics, and recurring schedules.

In modern web development, you often need to work with dates. For example, you might make billing cycles or fill dropdowns with months. So, finding the first day of each month between two dates is key. In this guide, we will show you how to get these dates. We mean the first day of every month. We will use native JavaScript and libraries like Moment.js, date-fns, and Luxon. You will also learn how to deal with odd cases, manage timezones, make things faster with generators, and put this method into reusable code for bigger projects.

Native vs External JavaScript Date Libraries

When changing dates, developers often decide between using native JavaScript tools or popular date libraries. Both ways have good and bad points.

Native JavaScript Advantages

✅ No Other Programs Needed: It has ready-to-use functions. You do not need to install another package.
⚡ Small and Fast: It does not make your app bigger with large libraries.
🔄 Can Change Directly: You can change a Date object directly. This can be useful in fast loops.

But native JavaScript can also be a bit strange and not always clear. For example, months start at zero (January is 0, December is 11). And changing dates directly can cause problems if you are not careful.

When to Use External Date Libraries

External libraries are useful when:

🌍 You need to handle timezones or format for many places.
🔁 You often change dates, or do many changes one after another.
🔒 You need to keep the original date as it is. This is so you do not change other things by accident.

Some good JavaScript date libraries are:

Moment.js: A classic library with many features.
date-fns: Modern, functional, and small.
Luxon: Does not change dates directly and understands timezones.

For the task of getting first-day-of-month dates, native JavaScript is strong enough. But if your project needs more complex things, like daylight saving time or different country settings, a library becomes very helpful.

Understanding JavaScript's Date Object

JavaScript’s Date object gives a basic way to work with dates and times. It has some odd parts. But knowing its basic behavior helps you write better code.

Key Characteristics

Zero-Indexed Months: new Date(2024, 0) creates January, not February.
Date Overflow Handling: If you add 13 months, JavaScript changes the year automatically.

const date = new Date(2024, 11, 1); // December 1, 2024
date.setMonth(date.getMonth() + 1); // January 1, 2025

Mutation Happens: Methods like setMonth, setDate, and setYear change the Date instance directly.
Time Component Defaults to Midnight: If not specified, a created date assumes 00:00:00.

Knowing these details helps your calculations stay exact when making a JavaScript tool to work with date ranges.

Basic Strategy: Loop Over the JavaScript Date Range

To get the first day of month for all months within two given dates:

Make the start date normal. It should be the first day of its month.
Loop until the end date is reached or gone past.
In each loop, add a new date to a list of results.
Add one month to the date each time the loop runs.

This makes sure you get dates like:

"2021-03-01T00:00:00.000Z"
"2021-04-01T00:00:00.000Z"
"2021-05-01T00:00:00.000Z"

JavaScript Code Example: First Day of Month

Let's put this plan into action using plain JavaScript.

function getFirstOfMonthDates(startDate, endDate) {
  const result = [];

  let start = new Date(startDate);
  let end = new Date(endDate);

  if (isNaN(start) || isNaN(end)) {
    throw new Error("Invalid date input.");
  }

  if (start > end) [start, end] = [end, start];

  const current = new Date(start.getFullYear(), start.getMonth(), 1);

  while (current <= end) {
    result.push(new Date(current)); // copy for immutability
    current.setMonth(current.getMonth() + 1);
  }

  return result;
}

Example Output

getFirstOfMonthDates("2023-01-10", "2023-04-30")
// [
//   2023-01-01T00:00:00.000Z,
//   2023-02-01T00:00:00.000Z,
//   2023-03-01T00:00:00.000Z,
//   2023-04-01T00:00:00.000Z
// ]

This function includes all dates. Both the start and end months are part of the output if they are in the range.

Edge Cases: Invalid or Reversed Inputs

Your code should be strong and work well in real situations.

Potential Errors

❌ Start date after end date
❌ Malformed strings like "abc" or unsupported formats
❌ Null or undefined values

Defensive Programming

Add checks in your function code:

if (isNaN(new Date(startDate)) || isNaN(new Date(endDate))) {
  throw new Error("Invalid input dates.");
}

Also, fix dates that are backwards:

if (start > end) {
  [start, end] = [end, start];
}

These checks make your function more reliable. They also make debugging easier when the code is live.

Using Moment.js, date-fns, and Luxon

Using known libraries can help deal with unusual situations, lessen bugs, and make the code easier to read.

Moment.js

Moment.js is not actively developed now, with newer libraries taking its place. But people still use it a lot.

function getFirstDaysWithMoment(start, end) {
  const moment = require('moment');
  const result = [];
  let current = moment(start).startOf("month");

  while (current.isSameOrBefore(end)) {
    result.push(current.clone().toDate());
    current.add(1, "month");
  }

  return result;
}

date-fns

People know it for its small, functional parts. It also helps keep code size small.

import { parseISO, startOfMonth, addMonths, isAfter } from 'date-fns';

function getFirstDaysDateFns(start, end) {
  const result = [];
  let current = startOfMonth(parseISO(start));
  const endDate = parseISO(end);

  while (!isAfter(current, endDate)) {
    result.push(current);
    current = addMonths(current, 1);
  }

  return result;
}

Luxon

A Moment.js team member made Luxon. It does not change dates directly and understands timezones.

const { DateTime } = require("luxon");

function getFirstDaysLuxon(start, end) {
  const result = [];
  let current = DateTime.fromISO(start).startOf("month");
  const endDate = DateTime.fromISO(end);

  while (current <= endDate) {
    result.push(current.toJSDate());
    current = current.plus({ months: 1 });
  }

  return result;
}

Each of these libraries has built-in tools to read, check, and change dates. This can save a lot of time fixing bugs.

Timezones and UTC Considerations

In JavaScript, the Date object is affected by your computer's timezone unless you say otherwise.

Common Pitfalls

Parsing ISO strings like "2023-01-01" might give different results based on your system settings.
Formatting with toString() shows the local timezone. This can cause confusion when saving or logging dates.

Best Practices

Always prefer new Date(YYYY, MM, DD) when creating logical dates.
Use toISOString() for sending data or logs:

const date = new Date(2024, 0, 1);
console.log(date.toISOString()); // Outputs 2024-01-01T00:00:00.000Z

For date calculations in backend programs or APIs, it's usually best to work in UTC. This helps make sure dates are the same for all users.

Formatting the Output Date Range

How you format the output depends on how you use it and who sees it.

Display Formats

For users: toLocaleDateString('en-US', { month: 'long', year: 'numeric' }) → "January 2024".
For logging or JSON APIs: toISOString() works well — "2024-01-01T00:00:00.000Z".
For dashboards: Format to "Jan 2024" using Intl.DateTimeFormat or external libraries.

Example:

result.map(date =>
  new Intl.DateTimeFormat("en-US", { year: "numeric", month: "short" }).format(date)
)
// Outputs: ["Jan 2024", "Feb 2024", "Mar 2024"]

Performance and Efficiency for Large Ranges

Getting first-of-month dates from years like 1990 to 2024 makes 408 entries. This is easy enough to do, but speed matters when you need to do more.

Optimization Tips

♻️ Use generators to get values only when needed.
🧊 Avoid in-place mutations unless controlled carefully.
🧪 Check how fast it is during stress tests for many items.

Generator-Based Lazy Evaluation

JavaScript generators make values only when asked. This saves memory.

function* getMonthlyFirsts(startDate, endDate) {
  let current = new Date(startDate.getFullYear(), startDate.getMonth(), 1);
  const end = new Date(endDate.getFullYear(), endDate.getMonth(), 1);

  while (current <= end) {
    yield new Date(current);
    current.setMonth(current.getMonth() + 1);
  }
}

Usage

for (let date of getMonthlyFirsts(new Date("2000-01-01"), new Date("2024-01-01"))) {
  console.log(date.toISOString());
}

This method is good for showing things as they come in. It is also good when you do not know the full date range ahead of time.

Use Cases for First Day of Month in JavaScript

Use this method any time you need to get dates between two dates at the start of each month.

Real-World Applications

🧾 Billing & Invoicing — Charge on the first of each month
📊 Reports & Dashboards — Show data grouped by month
📅 Calendar App — Let users pick months
🔁 Recurring Events — Schedule reminders or automation

Any program that works with months (for money, tasks, or data analysis) can use this method to get dates by month.

UI Tips: Making It User-Friendly

Making it easier to use when picking months needs good design on the front end.

Suggestions

Validate start and end dates before executing logic.
Show a calendar or dropdown with only months, not full dates.
Highlight current and future months differently from past.
Show small hints, previews, or summaries to explain what the user picked.

By making this feature focused on the user, you cut down errors and get users to interact more.

Creating a Reusable Utility Module

To avoid writing the same code in many parts of your app, make a separate JavaScript file.

// utils/dateUtils.js
export function getFirstOfMonthDates(startDate, endDate) {
  // reuse earlier implementation
}

Bonus: Unit Testing

// dateUtils.test.js
test("Returns correct number of months", () => {
  const result = getFirstOfMonthDates("2022-01-01", "2022-03-01");
  expect(result.length).toBe(3);
});

Also check for:

Reverse ranges
Leap years
Invalid inputs
Immutability checks

Build Once, Reuse Often

Finding the first day of each month in a JavaScript date range is a key tool for many programs. You can use JavaScript's built-in Date object. Or, you can pick from newer libraries like date-fns or Luxon. This lets you change the method to fit many needs. This includes showing dates on the screen or for backend billing. When you make it faster and build it with small, separate parts, you can use this feature in all your code as your project grows. Every JavaScript developer should know how to do this.