TheAlgorithms/JavaScript · error · Error

Invalid day value.

Error message

Invalid day value.

What it means

Thrown by checkDate() (called from parseDate) in Timing-Functions/ParseDate.js when date.day is less than 1 or greater than getMonthDays(date.month, date.year). It uses the GetMonthDays helper to bound the day to the real month length, so February in a leap year allows 29, otherwise 28. Generic Error indicating the day is out of range for the parsed month.

Source

Thrown at Timing-Functions/ParseDate.js:5

import { getMonthDays } from './GetMonthDays'

function checkDate(date) {
  if (date.day < 1 || date.day > getMonthDays(date.month, date.year)) {
    throw new Error('Invalid day value.')
  }
}

function parseDate(dateString) {
  const regex = /^(\d{1,2})\/(\d{1,2})\/(\d{4})$/

  const match = dateString.match(regex)

  if (!match) {
    throw new Error("Invalid date format. Please use 'dd/mm/yyyy'.")
  }

  const res = {
    day: parseInt(match[1], 10),
    month: parseInt(match[2], 10),
    year: parseInt(match[3], 10)
  }
  checkDate(res)

View on GitHub (pinned to 5c39e87a9a)

Solutions

1. Confirm you are passing dd/mm/yyyy order, since parseDate reads the first group as the day.
2. Validate the day against the month length before calling, or rely on parseDate and catch the error.
3. Use a native Date object with proper year/month/day for arithmetic, since this helper is for parsing a fixed format.
4. For February 29, ensure the year is a leap year (isLeapYear) before accepting day=29.

Example fix

// before
parseDate('31/04/2023') // April has 30 days, throws

// after
parseDate('30/04/2023') // valid last day of April

function parseDateSafe(str) {
  const d = parseDate(str) // parseDate already runs checkDate
  return d
}
// or pre-check the day against the month length before calling

const isPlausibleDayRange = (d, m, y) => d >= 1 && d <= getMonthDays(m, y)

try { parseDate('31/04/2023') } catch (e) { if (/Invalid day/.test(e.message)) { /* ask user to re-enter */ } else throw e }

When it happens

Trigger: Parsing a date whose day exceeds the month length: parseDate('31/04/2023') (April has 30 days) → day=31 > 30; parseDate('29/02/2023') (2023 not leap) → day=29 > 28; parseDate('00/01/2021') → day=0 < 1; parseDate('32/01/2021') → 32 > 31. Also note parseDate interprets match[1] as the DAY and match[2] as the MONTH (dd/mm/yyyy order).

Common situations: Mixing US (mm/dd/yyyy) and EU (dd/mm/yyyy) order — passing '02/30/2021' meaning Feb 30 in US order, but parseDate reads it as day=02, month=30 which throws Invalid Month Number first; allowing day=29 for February without checking the year; user-entered dates without a calendar widget.

Related errors

• ${creditCardString} is an invalid credit card number because
• Invalid Month Number.
• Invalid date format. Please use 'dd/mm/yyyy'.
• Invalid Triangle sides.
• Input is not a valid 2D matrix.

AI-assisted analysis of TheAlgorithms/JavaScript@5c39e87a9a (2026-08-13). Data as JSON: /api/errors/8ab510e99cb4e1b0. Report an issue: GitHub.