formatMonthDay

Signature

formatMonthDay(value: string, locale?: string | string[], options?: DateTimeFormatOptions): string

Return a localized string for a month-day (--MM-DD or MM-DD) using Intl options.

Formats as Temporal’s ECMA-402 PlainMonthDay format does: only month and day are kept, at noon UTC on the ISO date in the 1972 reference year (so --02-29 formats). The year is never printed.
Gregorian rule (GMT). Always formats through the Gregorian calendar, which names the same month and day as the ISO one. ICU formats calendar: "iso8601" without the locale’s month-day pattern (Node prints " 25", browsers Dezember 25 for de-DE), and PlainMonthDay#toLocaleString throws for a calendar mismatch. A -u-ca- subtag in locale is overridden. With a calendar option, the calendar Intl resolves must be gregory or iso8601, otherwise "". Intl ignores an unsupported value and uses the locale’s default, so { calendar: "foo" } gives "12/25" in en-US and "" in th-TH (Buddhist).
dateStyle. The month and day widths are the locale’s own dateStyle date pattern’s, as ECMA-402 (AdjustDateTimeStyleFormat) and native PlainMonthDay#toLocaleString take them: de-DE medium is 25.12., en-US medium Dec 25 and zh-CN long 12/25. Polyfill 0.5.1 maps each style to fixed widths instead, so its toLocaleString prints 25. Dez. for de-DE medium.
A year, weekday, era, time field or timeZoneName is dropped; only those fields with no month or day give "", except era and timeZoneName, which fall back to the defaults.
Returns "" for invalid input, a timeStyle, invalid options, null options or an ill-formed locale.

Parameter	Type	Description
`value`	`string`	ISO month-day string
`locale`	`string` \| `string[]`	optional BCP 47 locale identifier (default: runtime default), or a preference list of tags (ECMA-402)

options

optional Intl.DateTimeFormatOptions

Option	Type	Default	Description
`…Intl.DateTimeFormatOptions`	—	—	Every field of `DateTimeFormatOptions`, as ECMA-402 defines it.

localized month-day string or "" on invalid input

import { formatMonthDay } from "@northguild/gmt/plain/format";

formatMonthDay("--12-25", "en-US") // "12/25"

value

locale (optional)

output