addMonths

Adds months to a date.

Returns a new Date — the original is never mutated. When the resulting month has fewer days, JavaScript clamps to the next month (e.g. Jan 31 + 1 month → Mar 3). Use with caution. Returns null if the input is invalid.

Available since v2.0.0

Import

import { addMonths } from '@helpers4/date';

Signature

addMonths(date: DateLike, amount: number): Date | null

Parameters

Parameter	Type	Description
date	DateLike	The base date
amount	number	Number of months to add (negative to subtract)

Returns

Date | null — A new Date, or null if the input is invalid

Examples

addMonths

addMonths('2025-01-15', 1)    // => Date(2025-02-15)
addMonths('2025-01-31', 1)    // => Date(2025-03-03) — overflow
addMonths('2025-03-15', -1)   // => Date(2025-02-15)

Related Types

DateLike

A value that can be converted to a Date.

• Date — used as-is (validated for Invalid Date)
• number — treated as a timestamp (seconds or milliseconds, auto-detected); 0 is treated as invalid and produces null in ensureDate
• string — parsed via new Date(string)
• EpochMilliseconds — any object with a epochMilliseconds property (e.g. Temporal.Instant, Temporal.ZonedDateTime)

// TODO: When the Temporal API reaches Stage 4 and is available without // flags in all major runtimes, consider narrowing the union to the // concrete Temporal types for stricter type-checking.

type DateLike = Date | number | string | EpochMilliseconds

EpochMilliseconds

An object that exposes an epoch timestamp in milliseconds.

This structural type is satisfied by Temporal.Instant and Temporal.ZonedDateTime (and any future object that carries the same property), so callers can pass Temporal values without importing them.

interface EpochMilliseconds {
  epochMilliseconds: number;
}

Source

View source on GitHub