Date Difference Calculator (D-day)
Find out exactly how many days, weeks, months, or years lie between two dates. Use it for D-day countdowns to an anniversary, tracking how many days you have been together, contract periods, and any other date calculation you need.
Examples
From January 1, 2026 to December 31, 2026
The difference between the two dates is 364 days. Counting the start date as well, it is 365 days (one year).
100-day anniversary
If a couple started dating on January 1, 2026 and that day counts as day 1, the 100th day falls on April 10, 2026.
Contract period
A lease running from March 1, 2026 to February 29, 2028 is exactly two years (730 days).
FAQ
Should the start date be included in the difference?
It depends on the purpose. Stays and contract periods usually use the pure difference that excludes the start date, while anniversary counts such as 'days since we met' include the first day as day 1. This calculator shows both results.
How is a 100-day anniversary calculated?
The common convention is to count the first day as day 1. That means the 100th day falls 99 days after the starting date.
How is the difference in months calculated?
It is based on the calendar. For example, January 15 to March 15 is exactly two months. Because months have different lengths, the number of months is not directly proportional to the number of days.
Are leap years taken into account?
Yes. This calculator works on the actual calendar, so February 29 in leap years is handled correctly. A leap year is a year divisible by 4 that is either not divisible by 100 or divisible by 400.
Can it calculate working days (business days)?
This calculator shows the total number of days along with an approximate count of weekdays excluding weekends. Public holidays are not taken into account, so subtract them separately for an exact business-day count.
When you need date calculations
Everyday life is full of situations that call for date math: lease and rental contract periods, passport and visa validity, project deadlines, days left until an exam, how many days old a newborn is, couple anniversaries, and more.
In legal period calculations under Korean civil law, the first day is excluded by default (the 'first-day exclusion' principle). However, when a period starts at midnight, the first day is included.
Common date units
1 week = 7 days, 1 year = 365 days (366 in a leap year), 100 days ≈ 14 weeks 2 days, and 1,000 days ≈ 2 years 8 months 26 days.
On average a month is 30.44 days (365.25 ÷ 12), but calendar month calculations are usually done from the same day of the month to the same day of the next.
Related Calculators
Korean Age Calculator
Calculate your international age and Korean year age from your date of birth. This free age calculator follows Korea's 2023 age standardization law and also shows your zodiac sign and days until your next birthday.
Percentage Calculator
Find what percent one value is of another, calculate percentage increase or decrease, and get X% of any number. A free percentage calculator for discounts, raises, and ratios.