D-day, Age & Timestamp
D-day countdown, age, date difference with business days, and two-way Unix timestamp conversion.
🔒 All calculations happen only in your browser.
Four calculators in one panel
The tool groups four date jobs behind a single tab bar. D-day takes a target date and an optional label, then shows whether the day is still ahead (D-N), today (D-DAY), or already passed (D+N); for future dates it adds a live countdown in days, hours, minutes, and seconds that ticks once every second. Age turns a birth date into your international age, your Korean (counting) age, the total number of days you have been alive, and the days left until your next birthday. Date diff reports the gap between any two dates in days, whole weeks, and business days that skip Saturdays and Sundays — and the order you enter the dates in does not matter. Timestamp converts a Unix value into local, ISO 8601, and UTC time, and turns a date back into seconds and milliseconds.
When it earns its place
Reach for D-day when a fixed date is looming — an exam, a wedding, a product launch, a contract renewal — and you want the countdown on screen instead of in your head. The age tab is handy for filling in forms, checking eligibility cut-offs, or settling the perennial gap between someone's international age and their Korean age. Date diff suits planning and billing: measure a sprint in working days, count the weekdays left before a deadline, or track how long it has been since a launch or an anniversary. The timestamp tab is aimed at developers reading logs, database rows, or API responses, where dates often arrive as raw Unix numbers — paste one in and the human-readable time appears at once, or go the other way to build a query value.
Accurate, and private by design
Every calculation runs in your browser using JavaScript's built-in date engine, so the dates you type are never sent to a server or stored anywhere; once the page has loaded it keeps working offline. Times follow your device's local timezone, while the ISO 8601 and UTC outputs give you unambiguous values to copy into code or share across regions. The timestamp field detects its unit by length: ten digits or fewer are read as seconds, longer strings as milliseconds. One caveat worth knowing on the business-day count — it excludes weekends only, not public holidays, so for payroll or SLA work you may still need to subtract national holidays yourself. Everything else recalculates the instant you change an input, with nothing to submit and no sign-in.
Frequently asked questions
How is the D-day number and countdown worked out?
D-day compares your target date with today at midnight. A future date shows D-N (N days remaining) together with a live countdown in days, hours, minutes and seconds that refreshes every second; the target day itself shows D-DAY; and a past date shows D+N, the number of days since. Add an optional label to keep track of what you are counting down to.
What is the difference between "Age" and "Korean age"?
"Age" is your international age, which only increases after your birthday has passed each year. "Korean age" counts the year you were born as year one, so it is usually one to two years higher than the international value. The same tab also shows the total number of days you have been alive and the days left until your next birthday.
Does the timestamp tab handle both seconds and milliseconds?
Yes. It auto-detects by digit count — up to ten digits is read as seconds, anything longer as milliseconds — and shows the result as local time, ISO 8601 and UTC. You can also enter a date and time to get the Unix value back in both seconds and milliseconds. All conversions happen locally in your browser.