# Calculate the number of months between two dates

Count calendar month boundaries from start to end.

Canonical: https://1loc.dev/date-time/calculate-the-number-of-months-between-two-dates/
Version: 1.0.0
Runtime: JavaScript · Node.js 22 / modern browsers
Verification: Local fixtures; no external services
Updated: 2026-10-28

## Implementation

```javascript
function monthDiff({ start, end }) { const a = new Date(start), b = new Date(end); return Math.max(0, (b.getUTCFullYear() - a.getUTCFullYear()) * 12 - a.getUTCMonth() + b.getUTCMonth()); }
```

## Input

```json
{
  "start": "2026-01-15",
  "end": "2026-10-28"
}
```

## Expected output

```json
9
```

## Edge case

Input:
```json
{
  "start": "2026-10-28",
  "end": "2026-01-01"
}
```
Output:
```json
0
```

## Limits

- Days are ignored: January 31 to February 1 counts as one month. An end before the start returns 0.

The lab validates types and size before calling this function. Add equivalent validation at your own trust boundary.

## Source and license

Original: https://github.com/phuocng/1loc/blob/14b481cf60dff1620ab1bb99a43b1f68068cdd9b/contents/calculate-the-number-of-months-between-two-dates.mdx
Author: Nguyen Huu Phuoc and original contributors
Contributors: kwaimind
Source commit: 14b481cf60dff1620ab1bb99a43b1f68068cdd9b
Copyright (c) 2020 phuoc-ng
License: MIT — https://1loc.dev/licenses/
Maintainer: Oveyon
Modifications: Reads validated YYYY-MM-DD dates in UTC instead of local Date objects. Original MDX retained as inert source evidence.