Leap-Day Birthday

Key point

Yearly dates with no year

A month-day has no year to drift. In a year with no 29 February it becomes 28 February, and GMT tells you it moved.

A birthday, a renewal or an anniversary repeats every year, so it is stored without one: --02-29. monthDayToDate puts it in a year and, where the year has no such day, gives the nearest earlier one.

Type a date, pick a locale and a year, or press one of the presets. The plates show the stored form, how it reads in the locale and the date in your year. The table shows eight years in a row, and two alternatives beside the library’s answer. Section 2 prints every call made. Every date on the page is a library result.

1. A date with no year

PresetCustomA date of birth on 29 FebruaryA century yearThe short form, MM-DDA date-timeA day the month does not have

A full date of birth is read for its month and day only. In a common year the library gives 28 February.

A full date of birth is read for its month and day only. In a common year the library gives 28 February.A century year is a leap year only when it divides by 400, so 2100 has no 29 February and 2104 does.The leading dashes are optional on the way in. What is stored always has them.A date-time is read for its month and day. The time never moves the day.Thirty February exists in no year, so no reader accepts it and nothing below is guessed.

Birthday or yearly date

Localeenen-USen-GBde-DEfr-FRes-ESit-ITpt-PTsv-SEis-ISzh-CNzh-TWja-JPko-KRar-SAhe-ILru-RUtr-TR

Year

202619262126

Stored as

⟨ NO SIGNAL ⟩+275760-09-13⟨ NO SIGNAL ⟩

Reads as

⟨ NO SIGNAL ⟩September 28⟨ NO SIGNAL ⟩

In that year

⟨ NO SIGNAL ⟩+275760-09-13⟨ NO SIGNAL ⟩

The date in eight consecutive years

Year	gmt	Verdict	Year pasted on (naive)	1 March rule (your policy)
-	⟨ NO SIGNAL ⟩+275760-09-13⟨ NO SIGNAL ⟩	no datekeptmoved earliermoved laterno date	⟨ NO SIGNAL ⟩+275760-09-13⟨ NO SIGNAL ⟩	⟨ NO SIGNAL ⟩+275760-09-13⟨ NO SIGNAL ⟩keptmoved earliermoved laterno date
-	⟨ NO SIGNAL ⟩+275760-09-13⟨ NO SIGNAL ⟩	no datekeptmoved earliermoved laterno date	⟨ NO SIGNAL ⟩+275760-09-13⟨ NO SIGNAL ⟩	⟨ NO SIGNAL ⟩+275760-09-13⟨ NO SIGNAL ⟩keptmoved earliermoved laterno date
-	⟨ NO SIGNAL ⟩+275760-09-13⟨ NO SIGNAL ⟩	no datekeptmoved earliermoved laterno date	⟨ NO SIGNAL ⟩+275760-09-13⟨ NO SIGNAL ⟩	⟨ NO SIGNAL ⟩+275760-09-13⟨ NO SIGNAL ⟩keptmoved earliermoved laterno date
-	⟨ NO SIGNAL ⟩+275760-09-13⟨ NO SIGNAL ⟩	no datekeptmoved earliermoved laterno date	⟨ NO SIGNAL ⟩+275760-09-13⟨ NO SIGNAL ⟩	⟨ NO SIGNAL ⟩+275760-09-13⟨ NO SIGNAL ⟩keptmoved earliermoved laterno date
-	⟨ NO SIGNAL ⟩+275760-09-13⟨ NO SIGNAL ⟩	no datekeptmoved earliermoved laterno date	⟨ NO SIGNAL ⟩+275760-09-13⟨ NO SIGNAL ⟩	⟨ NO SIGNAL ⟩+275760-09-13⟨ NO SIGNAL ⟩keptmoved earliermoved laterno date
-	⟨ NO SIGNAL ⟩+275760-09-13⟨ NO SIGNAL ⟩	no datekeptmoved earliermoved laterno date	⟨ NO SIGNAL ⟩+275760-09-13⟨ NO SIGNAL ⟩	⟨ NO SIGNAL ⟩+275760-09-13⟨ NO SIGNAL ⟩keptmoved earliermoved laterno date
-	⟨ NO SIGNAL ⟩+275760-09-13⟨ NO SIGNAL ⟩	no datekeptmoved earliermoved laterno date	⟨ NO SIGNAL ⟩+275760-09-13⟨ NO SIGNAL ⟩	⟨ NO SIGNAL ⟩+275760-09-13⟨ NO SIGNAL ⟩keptmoved earliermoved laterno date
-	⟨ NO SIGNAL ⟩+275760-09-13⟨ NO SIGNAL ⟩	no datekeptmoved earliermoved laterno date	⟨ NO SIGNAL ⟩+275760-09-13⟨ NO SIGNAL ⟩	⟨ NO SIGNAL ⟩+275760-09-13⟨ NO SIGNAL ⟩keptmoved earliermoved laterno date

⟨ NO SIGNAL — invalid input ⟩: no reader accepted this. A month-day is --MM-DD or MM-DD, and a date or a date-time is read for its month and day. A day the month does not have, such as --02-30, is refused.⟨ NO SIGNAL — invalid input ⟩: the month has no such day, so no reader accepts it and nothing below is guessed.⟨ NO SIGNAL — invalid input ⟩: the year must be a whole number the library can place, from -271821 to 275760. The stored month-day above is still read.Where a year has no 29 February, the library gives the nearest earlier day, 28 February. The pasted year is refused, and a 1 March rule is a policy some contracts use: it is built here from library calls, and gmt has no such mode.This day exists in every year, so the library keeps it. Only 29 February is ever moved, and the pasted year and the 1 March rule agree.

2. The calls

Every call behind the plates and the first row

isValidMonthDay("2024-02-29T08:30:00"); // false
parseMonthDay("2024-02-29T08:30:00"); // ""
parseMonthDayFromDate("2024-02-29T08:30:00"); // ""
parseMonthDayFromDateTime("2024-02-29T08:30:00"); // "--12-25"
formatMonthDay("--12-25", "zh-TW", { month: "long", day: "numeric" }); // "December 25"
monthDayToDate("--12-25", -271821); // "+275760-09-13"
parseMonthDayFromDate("+275760-09-13"); // "--12-25"
areMonthDaysEqual("--12-25", "--12-25"); // false
compareMonthDays("--12-25", "--12-25"); // 1
isValidDate("-271821-12-25"); // false
addDate("+275760-09-13", { days: 1 }); // "+275760-09-13"
parseMonthDayFromDate("+275760-09-13"); // "--12-25"
areMonthDaysEqual("--12-25", "--12-25"); // false
compareMonthDays("--12-25", "--12-25"); // 1

Stored as. parseMonthDay reads --MM-DD or MM-DD. A full date of birth goes through parseMonthDayFromDate, and a date-time through parseMonthDayFromDateTime. Each returns the canonical --MM-DD, or an empty string when it cannot read the input.

Reads as. formatMonthDay writes the month-day in a locale, with no year to invent.

gmt. monthDayToDate gives the date in the year. For --02-29 in a common year the answer is 28 February.

Verdict. areMonthDaysEqual says whether the date kept its month and day, and compareMonthDays says whether it moved earlier or later.

Year pasted on. Pasting the year onto the stored string and asking isValidDate is what a caller without a month-day type does. For 2023-02-29 the answer is no, and the cell reads NO SIGNAL. That is the failure this column shows, not a result of GMT.

1 March rule. Some contracts move the date to 1 March instead. That is a policy, and GMT has no mode for it: the column builds it from addDate on the library’s answer. Pick the rule your own agreement states and write it in your code.

See month-day values and a 29 February birthday in a common year.

Worth trying:

• A date of birth on 29 February — read for its month and day, and moved in most years.
• A century year — 2100 is not a leap year and 2104 is.
• 03-15, the short form — stored as --03-15 and kept in every year.
• A date-time — the time never moves the day.
• --02-30 — no reader accepts it, and nothing is guessed.