# Get the first date in the month of a date

Set the UTC day of the month to 1.

Canonical: https://1loc.dev/date-time/get-the-first-date-in-the-month-of-a-date/
Version: 1.0.0
Runtime: JavaScript · Node.js 22 / modern browsers
Verification: Local fixtures; no external services
Updated: 2026-10-28

## Implementation

```javascript
function firstDateOfMonth(date) { const d = new Date(date); d.setUTCDate(1); return d.toISOString().slice(0, 10); }
```

## Input

```json
"2026-10-28"
```

## Expected output

```json
"2026-10-01"
```

## Edge case

Input:
```json
"2024-02-29"
```
Output:
```json
"2024-02-01"
```

## Limits

- Works on UTC calendar dates; the original built a local Date at midnight.

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/get-the-first-date-in-the-month-of-a-date.mdx
Author: Nguyen Huu Phuoc and original contributors
Contributors: See original repository history
Source commit: 14b481cf60dff1620ab1bb99a43b1f68068cdd9b
Copyright (c) 2020 phuoc-ng
License: MIT — https://1loc.dev/licenses/
Maintainer: Oveyon
Modifications: Works on UTC calendar dates instead of a local Date at midnight. Original MDX retained as inert source evidence.