getWeekOfMonth

Signature

getWeekOfMonth(value: string, locale: string | string[]): number | null

Return the 1-based row value falls on within its month’s calendar grid, using locale’s first day of week.

Week 1 is the row containing the 1st of the month, even when that row is a partial week. This is GMT’s own rule, not a standard’s: a month grid draws every day of the month, so its first row always holds the 1st.
Not CLDR’s week-of-month field (W, UTS #35): that count applies the locale’s minDays, so a short leading week may not count as part of the month.
Comparison: date-fns’s getWeekOfMonth returns the same number for the same first day of week.
Distinct from ISO weekOfYear: this counts rows within a single month’s grid, reset every month, rather than weeks since Jan 1.
Resolves the locale’s first day of week via Intl.Locale.prototype.weekInfo.
Returns null if value or locale is invalid.

Parameter	Type	Description
`value`	`string`	ISO PlainDate string
`locale`	`string` \| `string[]`	BCP 47 locale tag (e.g. “en-US”, “fr-FR”), or a preference list of tags (ECMA-402; the first with locale data is read). Required: omitted, or an empty list (which ECMA-402 would resolve to the host default), returns null

1-based week-of-month row, or null on invalid input

import { getWeekOfMonth } from "@northguild/gmt/plain/calculate";

getWeekOfMonth("2024-02-01", "en-US") // 1