freecodecamp--freecodecamp/curriculum/challenges/english/blocks/daily-coding-challenges-javascript/6a2037a68a0bc2aef0006003.md

id, title, challengeType, dashedName

id	title	challengeType	dashedName
6a2037a68a0bc2aef0006003	Challenge 341: Birthday Countdown	28	challenge-341

--description--

Given today's date and a birthday, return the number of days until the person's next birthday.

• Today's date is given as a string in "YYYY-MM-DD" format, with leading zeros, for example: "2026-07-16".
• The birthday is given as a string in "M/D" format, without leading zeros, for example: "9/7".
• If today is their birthday, return the number of days until their next birthday (not 0).
• Leap years should be accounted for.

--hints--

daysUntilBirthday("2026-07-16", "9/7") should return 53.

assert.equal(daysUntilBirthday("2026-07-16", "9/7"), 53);

daysUntilBirthday("2026-07-16", "3/22") should return 249.

assert.equal(daysUntilBirthday("2026-07-16", "3/22"), 249);

daysUntilBirthday("2026-07-16", "7/16") should return 365.

assert.equal(daysUntilBirthday("2026-07-16", "7/16"), 365);

daysUntilBirthday("2024-02-28", "3/1") should return 2.

assert.equal(daysUntilBirthday("2024-02-28", "3/1"), 2);

daysUntilBirthday("2023-04-24", "12/30") should return 250.

assert.equal(daysUntilBirthday("2023-04-24", "12/30"), 250);

daysUntilBirthday("2024-03-01", "2/29") should return 1460.

assert.equal(daysUntilBirthday("2024-03-01", "2/29"), 1460);

daysUntilBirthday("2096-03-01", "2/29") should return 2920.

assert.equal(daysUntilBirthday("2096-03-01", "2/29"), 2920);

--seed--

--seed-contents--

function daysUntilBirthday(today, birthday) {

  return today;
}

--solutions--

function daysUntilBirthday(today, birthday) {
  const [year, month, day] = today.split("-").map(Number);
  const [bMonth, bDay] = birthday.split("/").map(Number);

  const todayDate = new Date(year, month - 1, day);

  function isLeapYear(y) {
    return (y % 4 === 0 && y % 100 !== 0) || y % 400 === 0;
  }

  function nextValidBirthday(fromYear) {
    for (let y = fromYear; y <= fromYear + 8; y++) {
      if (bMonth === 2 && bDay === 29 && !isLeapYear(y)) continue;
      const date = new Date(y, bMonth - 1, bDay);
      if (date > todayDate) return date;
    }
  }

  const nextBirthday = nextValidBirthday(year);
  const diff = nextBirthday - todayDate;
  return Math.round(diff / (1000 * 60 * 60 * 24));
}