ow many days until christmas 999999999999

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How many days until Christmas?

Enter year (leave empty for next Christmas)

Enter a year and click Calculate.

Uses exact calculation when possible. Very large years use a safe approximation.

(function(){ const calcBtn = document.getElementById(‘calcBtn’); const resetBtn = document.getElementById(‘resetBtn’); const yearInput = document.getElementById(‘christYear’); const resultBox = document.getElementById(‘resultBox’); // Safe JS Date boundary – keep a conservative range to avoid unreliable Date behavior const SAFE_MIN_YEAR = -1000000; const SAFE_MAX_YEAR = 1000000; function daysBetweenDates(d1, d2) { // Use UTC midnight to avoid timezone offset issues const msPerDay = 24 * 60 * 60 * 1000; const utc1 = Date.UTC(d1.getUTCFullYear(), d1.getUTCMonth(), d1.getUTCDate()); const utc2 = Date.UTC(d2.getUTCFullYear(), d2.getUTCMonth(), d2.getUTCDate()); return Math.round((utc2 – utc1) / msPerDay); } function approxDaysForFarYear(targetYear, today) { const currentYear = today.getUTCFullYear(); // days remaining this year until Dec 25 UTC const dec25ThisYear = new Date(Date.UTC(currentYear, 11, 25)); const daysUntilThisDec25 = daysBetweenDates(today, dec25ThisYear); const yearsDiff = targetYear – currentYear; // average Gregorian year length const avgYear = 365.2425; // approximate days to target Dec 25 const approxDays = Math.round(yearsDiff * avgYear + daysUntilThisDec25); return approxDays; } function calculate() { const raw = yearInput.value.trim(); const now = new Date(); const todayUTC = new Date(Date.UTC(now.getUTCFullYear(), now.getUTCMonth(), now.getUTCDate())); let targetYear; if (raw === ”) { // choose upcoming Christmas: if today is after Dec 25 (or on), choose next year const thisYearDec25 = new Date(Date.UTC(now.getUTCFullYear(), 11, 25)); targetYear = now > thisYearDec25 ? now.getUTCFullYear() + 1 : now.getUTCFullYear(); } else { // parse integer (can be very large) // Use Number for reasonable ranges, but for extremely large numbers we still treat as BigInt-like via string // We’ll try to convert to Number for safe range check; if not safe, we treat as far year. if (!/^-?d+$/.test(raw)) { resultBox.innerHTML = ‘Invalid year. Enter digits only.‘; return; } // parse as integer string const parsed = Number(raw); if (Number.isFinite(parsed) && Math.abs(parsed) <= 1e12) { targetYear = Math.trunc(parsed); } else { // extremely large – use approximation path // convert to BigInt for sign-check maybe, but we only need sign const sign = raw.startsWith('-') ? -1 : 1; // if negative or absurdly large, still attempt approximation relative to current year using string subtraction // Convert both to BigInt if possible try { const targetBig = BigInt(raw); const currentBig = BigInt(now.getUTCFullYear()); const yearsDiffBig = targetBig – currentBig; // If absolute difference exceeds 1e9, we'll just approximate using avg year const absDiff = yearsDiffBig BigInt(1e9)) { // huge difference const approx = approxDaysForFarYear(Number(now.getUTCFullYear()) + Number((yearsDiffBig > 0 ? 1 : -1) * BigInt(0)), todayUTC); // We won’t compute precise yearsDiff as Number (unsafe), instead compute using string math: // compute yearsDiff as decimal by subtracting string lengths: // Simpler: show approximate message and compute approxDays via avgYear * difference (use BigInt -> string conversion) // We’ll compute approxDays using BigInt yearsDiff when it’s not too large to convert to Number. resultBox.innerHTML = `

Approximation for year ${raw}

The year is outside JavaScript’s reliable date range. Showing an approximate number of days using average year length (365.2425).

Approx. days: ${approx.toLocaleString()}

`; return; } else { // safe to convert to Number for calculation targetYear = Number(targetBig); } } catch (e) { // BigInt not available or parse failed -> fallback to approximation const approx = approxDaysForFarYear(Number(now.getUTCFullYear()), todayUTC); resultBox.innerHTML = `

Approximation for year ${raw}

The year is outside JavaScript’s reliable date range. Showing an approximate number of days using average year length (365.2425).

Approx. days: ${approx.toLocaleString()}

`; return; } } } // At this point targetYear is a Number if (targetYear >= SAFE_MIN_YEAR && targetYear <= SAFE_MAX_YEAR) { // build Dec 25 UTC of targetYear const targetDate = new Date(Date.UTC(targetYear, 11, 25)); const days = daysBetweenDates(todayUTC, targetDate); if (days === 0) { resultBox.innerHTML = `

It’s Christmas today! 🎄

`; } else if (days > 0) { resultBox.innerHTML = `

${days.toLocaleString()} day(s) until Christmas ${targetYear}.

`; } else { // negative -> Christmas already passed; show absolute and suggest next-year resultBox.innerHTML = `

Christmas ${targetYear} passed ${Math.abs(days).toLocaleString()} day(s) ago.

Days until next Christmas (${targetYear + 1}): ${daysBetweenDates(todayUTC, new Date(Date.UTC(targetYear + 1,11,25))).toLocaleString()}

`; } } else { // outside safe range -> approximation const approx = approxDaysForFarYear(targetYear, todayUTC); resultBox.innerHTML = `

Approximation for year ${targetYear}

The requested year is outside JavaScript’s reliable date range. Using average Gregorian year (365.2425 days) for an approximate result.

Approx. days: ${approx.toLocaleString()}

`; } } calcBtn.addEventListener(‘click’, calculate); resetBtn.addEventListener(‘click’, function(){ yearInput.value = ”; resultBox.innerHTML = ‘Enter a year and click Calculate.’; }); // allow Enter key yearInput.addEventListener(‘keydown’, function(e){ if (e.key === ‘Enter’) { calculate(); } }); })();

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