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100D-day Calculator | Anniversaries and Months Between Dates
Count down to a date, find your 100-day and 1000-day anniversaries, or measure the months between two dates — with an option to count the first day as day 1.
How to use
Pick a mode
Pick a tab: D-day, anniversaries, or months between dates.
Enter dates
Enter the target or start date — results update instantly.
Check anniversaries
On the anniversary tab, choose whether day 1 is the start date.
Real-world scenarios
The same two dates can differ by a day depending on whether the first day counts. Anniversaries count it; legal periods usually do not.
Plan 100-day and 1000-day anniversaries ahead of time.
Check a baby’s age in months and days.
Track days and weeks left before exams or trips.
Date rules
Anniversaries count the first day as day 1 (optional). D-day and months use the difference between two dates.
| Jan 31 + 1 month = Feb 28 (29 in leap years): if the day does not exist, the month’s last day is used. |
| Months count how many times the start day recurs; remaining days are added. Mar 15 → Oct 27 = 7 months 12 days. |
Frequently asked questions
When is the 100-day anniversary?
Counting the start date as day 1, it is the start date + 99 days.
Is D-0 the same as D-Day?
Yes. The target date is D-Day; the day before is D-1 and the day after is D+1.