How to Count Anniversary Days: Working Out 100 Days, 1,000 Days and Other Milestones
When you count “what day is today” from the day you started dating or joined a company, the answer changes by one depending on whether you include the first day. This guide explains the difference between the two ways of counting, lists the milestones people like to celebrate, and shows how to set up a countdown to an anniversary in your browser.
“Day N” and “N days later” are one day apart
There are two ways to count days. One treats the starting day as day 1; the other treats it as day 0 and counts days elapsed. Starting from April 1, April 10 is day 10 in the first method but 9 days in the second. That single day is what splits an anniversary into two possible dates.
| Method | Starting day | Date of the 100th day | Where it's used |
|---|---|---|---|
| Starting day is day 1 | Day 1 | July 9 | Anniversaries, memorial services, traditional calendars |
| Count days elapsed | Day 0 | July 10 | Period calculations, contracts, tool displays |
Neither one is “correct”. What matters is agreeing on the method with the person you're celebrating with. If one of you counts the first day and the other counts elapsed days, your 100-day anniversary dates will be a day apart and you'll miss each other. Deciding up front solves it. The idea of counting one end versus both ends, and how to count across months, is explained in detail in how to calculate the days between dates.
Popular milestones to celebrate
Day-count milestones arrive at slightly different times from yearly anniversaries, and that offset is part of the fun of counting in days. If you want the yearly kind, counted in years and months, see the guide to counting “Nth anniversaries”.
| Milestone | Roughly | Notes |
|---|---|---|
| 100 days | About 3.5 months | The classic first anniversary |
| 365 days | 1 year | One day before the 1st anniversary if a leap day falls in between |
| 500 days | About 1 year 4 months | A handy marker between the 1st and 2nd anniversaries |
| 1,000 days | About 2 years 9 months | Comes about 3 months before the 3rd anniversary |
| 2,000 days | About 5 years 6 months | Just after the 5th anniversary |
| 3,000 days | About 8 years 2 months | Milestones start to spread out |
| 5,000 days | About 13 years 8 months | Between the 13th and 14th anniversaries |
| 7,300 days | About 20 years | 365 days × 20. About 5 days before the real 20th anniversary |
| 10,000 days | About 27 years 4 months | Counting from a birth date, it falls at age 27 |
365 days and the first anniversary don't always match because of leap years. If a leap day falls between the start and the same date next year, 365 days later is the day before the anniversary. It's a small detail, but for people who celebrate in days it means two separate days to enjoy.
How to set up a countdown
- 1
Open the anniversary tab
In the age & anniversary counter, choose the “Anniversary count” tab. Separate from the age tab, this mode lets you use any date as the starting point.
- 2
Enter a date and a name
Enter the date you want to count from, such as the day you started dating, your first day at work or your favorite artist's debut. Names can be up to 40 characters, so label each one clearly if you track several anniversaries.
- 3
Check days remaining and days elapsed
A future date shows the days remaining; a past date shows the days elapsed. Alongside the number of days you also see it as “X years, Y months, Z days” and roughly in weeks.
- 4
Look at the milestone list
The 3 most recent past milestones and the next 8 upcoming ones are listed. You can see at a glance which anniversary comes next and how many days away it is.
- 5
Copy the share URL
The date and name are stored in the URL, so sending it to someone opens the same screen for them. Your input is only included in the URL and is not sent to a server.
Counting the days since you were born
Use a birth date as the starting point and you get the number of days someone has been alive. Birthdays come every year, but day-count milestones come only once in a lifetime. As an excuse to celebrate family and friends, they are arguably even more special.
- 10,000 days: around age 27 years 4 months. A nice milestone to close out your twenties
- 15,000 days: around age 41
- 20,000 days: around age 54 years 9 months
- 25,000 days: around age 68 years 5 months
- 30,000 days: around age 82
In Japan a baby's 100th day is widely celebrated with okuizome, the first-meal ceremony, but few people pay attention to day 1,000 or 2,000 after that. Using them as an excuse to take photos leaves you with a record of growth at even intervals.
Things you can count as anniversaries
- The day you started dating, the day you registered your marriage, your wedding day
- Your first day at a company, the day you went independent, the day you opened a shop
- Your favorite artist's debut, a group's formation date, the day you went to your first concert
- The day you welcomed a pet, the day you moved
- The day you quit smoking or started studying. The number of days you've kept it up is motivating in itself
Enter a future date and it becomes a countdown. How many days until the exam, the departure date of a trip, or your favorite artist's concert? The same tool handles both days elapsed and days remaining.
Easy-to-miss details and what to decide in advance
| Tricky point | How to decide |
|---|---|
| Whether to include the first day | Agree with the other person. The tool shows days elapsed |
| Monthly anniversaries | If you start on the 31st, some months end on the 30th. Move it to the month end or read it as the end of the previous month |
| Starting on a leap day | Decide in advance whether to celebrate on February 28 or March 1 in common years |
| Counting with someone in another time zone | Decide whose country's date is the reference. The tool calculates dates in Japan time |
These may look like small details, but anniversaries come around every year and every month. Decide once at the beginning and you won't have to think about it again. Skip it, and you'll find yourselves checking which date it was every time. If you want to count a baby's milestones in days, such as the 100th day after birth or the half birthday, a guide focused on counting days after birth and age in months will suit you better.
FAQ
- When is our 100th day together?
- If you started on April 1, it is July 9 if you count the first day as day 1, or July 10 if you treat the start as day 0 and count elapsed days. Because there are two answers, decide which way you'll count before the day comes. The age & anniversary counter displays elapsed days.
- How long is 1,000 days in years and months?
- About 2 years and 9 months. It arrives roughly 3 months before the 3rd anniversary, so you can celebrate it separately. 2,000 days is about 5 years 6 months and 3,000 days about 8 years 2 months; the gaps between milestones gradually widen.
- How old are you at 10,000 days since birth?
- Around 27 years and 4 months, give or take a few days depending on how leap years fall. After that, 15,000 days is around 41, 20,000 days around 54 years 9 months and 30,000 days around 82. Enter your date of birth to see the exact date and the days remaining.
- Are the anniversaries I enter saved on a server?
- No. All calculations happen in your browser, and the dates and names you enter are only included in the share URL. Unless you give that URL to someone, nothing you enter leaves your device.
Tools in this article
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