Date & Age Calculator
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Sunday, October 18, 2026
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Four date questions in one place: how long until a date, how old someone is exactly, how many days lie between two dates, and what date falls a given number of days from now.
Two questions look similar and are not. "How many days between these dates?" is subtraction. "How old is this person?" is a calendar rule, and Korea uses more than one: 만 나이 counts completed years and changes on your birthday, while 연 나이 is simply the current year minus your birth year and changes for everyone at midnight on 1 January. The same person can be two different ages on the same day depending on which one a form is asking for.
This page keeps them separate rather than picking one for you, and does the same for date arithmetic: a plain day count, a count that skips weekends for working days, and a D-day that tells you whether the day itself is included. Leap years are handled by the calendar rather than by multiplying by 365, so a span crossing 29 February comes out right.
Business day counts are where a shared expectation quietly fails to exist. Excluding weekends is uncontroversial; excluding public holidays is not, because the holidays differ by country and, in Korea, several move each year with the lunar calendar. A count here excludes weekends and says so, rather than embedding one country calendar and being wrong everywhere else. For a deadline that genuinely depends on holidays — a contractual notice period, a filing date — the count is a starting point to adjust rather than an answer to quote.
Adding months is the operation that has no single correct answer. One month after 31 January is 28 February in most systems, because 31 February does not exist and clamping to the end of the month is the least surprising rule — but it makes the operation asymmetric: adding a month and then subtracting one does not always return you to the day you started on. Adding days has no such ambiguity, which is why a period defined in days is easier to reason about than one defined in months whenever the exact date matters.
A D-day count is off by one more often than any other calculation on this page, because two conventions are both in use. Counting the day itself makes the day of an event D-day and the day before D-1; not counting it makes the event D+1. Korean usage generally includes the day, which is why the option is here and defaults the way it does. Age has the same shape: 만 나이 counts completed years and changes on the birthday, while 연 나이 changes for everybody at midnight on 1 January.
International age counts completed years since birth and increases on your birthday. The year-based age is simply the current year minus the birth year. Korea moved to international age in law in June 2023, but the year-based number still appears in school and conscription contexts.
Yes. Day counts use real calendar arithmetic, so 29 February and leap years are handled correctly.
Because months differ in length. Adding one month to 31 January cannot produce 31 February, so the result rolls into March — the standard behaviour for calendar arithmetic.