Count the days between two dates, add or subtract days, count down to any event in D-day style, and find your 100-day and 1,000-day anniversaries.
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Dates are calendar dates with no time of day, so time zones and daylight-saving changes do not affect the counts. “Today” uses your device’s date.
How to use the D-day and date calculator
Choose one of four tools from the tabs at the top:
- Days between dates – count the exact number of days, weeks, months, and weekdays between any two dates.
- Add or subtract days – find the date that falls a certain number of days, weeks, months, or years before or after another date.
- D-day countdown – see how many days are left until an event, written in the popular “D-30” style.
- 100-day milestones – list the 100th, 200th, 1,000th, and other milestone days from a start date, such as a baby’s birth, a relationship, or a new habit.
The start date and “count from” date fill in with today’s date automatically, so for most questions you only need to enter one date.
Worked examples
- How many days from New Year’s Day to Christmas 2026? From January 1 to December 25, 2026 is 358 days: 11 months and 24 days, or 51 weeks and 1 day. 256 of those days are weekdays (Monday to Friday). Tick “Include the end date” and the count becomes 359 days.
- What date is 90 days from October 28, 2026? Add 90 days and the answer is Tuesday, January 26, 2027. This is handy for return windows, visa limits, and 90-day probation periods.
- How many days until Christmas? On October 28, 2026, the countdown reads D-58: 58 days, or about 8 weeks and 2 days.
- When is a 100-day anniversary? If a relationship or a baby’s life started on January 1, 2026, the 100th day is April 10, 2026 when the first day counts as day 1, the usual way in Korea. Counted as “100 days after” January 1, it would be April 11.
What does “D-day” mean?
“D-day” first meant the day a military operation begins. It is best known as June 6, 1944, the start of the Allied landings in Normandy. Today the term is used for any important date. In Korea, D-day countdowns are part of everyday life: students count down to the 수능 (the College Scholastic Ability Test), couples count to anniversaries, and workers count to vacations.
The format is simple:
- D-10 means the event is 10 days away.
- D-Day means the event is today.
- D+10 means the event was 10 days ago.
Why 100-day anniversaries are counted differently in Korea
In Korea, people celebrate 백일 (baek-il), a baby’s 100th day, and couples often celebrate their 100th, 200th, and 1,000th days together. These are counted with the first day as day 1, so the 100th day is 99 days after the start date. Many Western date calculators count “100 days after” instead, which lands one day later. Our milestone tool lets you choose either method with one checkbox, so the date always matches the way you and your family count.
Common questions
Does the calculator count leap years correctly?
Yes. It counts real calendar days, so February 29 is included in leap years, such as 2028.
Why might my count be off by one day?
It depends on whether you count the start date, the end date, or both. By default, the “days between” tool counts the gap between the dates, the way most people count nights at a hotel. If you want to count both the first and last day, as you would for an event that runs from Monday to Friday, tick “Include the end date.”
Are public holidays excluded from weekdays?
No. Holidays differ from country to country and year to year, so the weekday count includes every Monday to Friday. Subtract any holidays that apply to you.
What happens when I add one month to January 31?
Months have different lengths, so the calculator uses the last day of the shorter month. January 31 plus one month is February 28 (or February 29 in a leap year).
Related tools
- Age Calculator – exact age in years, months, and days
- Time Zone Explorer – what time and date it is anywhere in the world
- Calculator – quick everyday arithmetic