Find out exactly how many days remain until an important date, or how many days have passed since one. Handy for exam countdowns, anniversaries and project deadlines.
Two conventions collide here. Counting down, the target day itself is day zero. Counting up — "how many days has it been" — the day after the start date is day one, which is how school, military and anniversary counts are usually kept.
Counting the start day as day one — common in institutional and military contexts. The first day is "day 1".
Counting the start day as zero — the usual D-day convention. The event itself is D-day, the day before is D-1.
The ambiguity bites on the D+ side. Counting an anniversary from day one makes "100 days" the start date plus 99; counting from zero makes it plus 100. One day apart, and people argue about it.
This tool shows both. Agreeing on the basis first is what actually resolves it.
100 days together is conventionally counted with day one as the start, so start date plus 99.
One year is counted by date, not by 365 days. A leap year makes those differ by a day.
A baby's 100th day counts from birth as day one, so birth date plus 99.
The first birthday is by date, one year later.
"N days later" and "the same date N years later" are different calculations, and anniversaries almost always mean the second.
Exam countdowns conventionally treat the day itself as D-day (zero), so D-30 means thirty days before.
A planning caution: D-30 does not mean thirty days of study. The exam day and the day before go to review, and weekends differ in available energy.
This tool also shows the number of weekends remaining, which for anyone who cannot study on weekdays is much closer to the real figure.
Leap years are divisible by four, except centuries, except centuries divisible by 400. 2000 was a leap year; 1900 was not.
A 29 February birthday does not exist in common years, and jurisdictions differ on whether to use 28 February or 1 March.
Adding months at month end clamps: "one month after 31 January" has no 31 February, so it lands on the 28th or 29th. Because of that, going forward a month and back again does not always return you to the start.
Daylight saving is irrelevant for whole-day counts, but a day can be 23 or 25 hours long when times are involved.
Making the remaining time visible before an exam, a talk or a deadline.
Anniversaries you need advance notice of in order to prepare.
Administrative dates: contract expiry, visa expiry, warranty periods. Dates that cost money when missed.
Backward planning, deciding from a deadline when a draft is due. Entering the intermediate dates shows immediately whether the plan is realistic.
When coordinating an anniversary, agree the counting basis first. That single disagreement causes most of the arguments.
For shared deadlines, write the actual date alongside any D-number. "D-7" points at a different day depending on when it was calculated.
Everything is computed in the browser and no dates are transmitted.
The calculator counts the whole days between today and your chosen date. You can pick whether the start day counts as day zero or day one.
Yes. Choose a date in the past and the tool shows how many days have elapsed, so you can track anniversaries or milestones.
Yes — set the start date in the past and the tool counts elapsed days, so you can track 100-day or yearly anniversaries just as easily as countdowns.
What this tool bases its numbers on, and how far those numbers go.
Whole-day difference between the two calendar dates, taken from the dates alone and not from the clock. Day-zero basis: elapsed days = target − start. Day-one basis: elapsed days = target − start + 1, so the start date itself is day 1.From 2026-08-26 to 2026-12-25 is 121 days, shown as D-121. Counting elapsed days on the day-one basis, day 100 from 2026-08-26 is 2026-12-03 (start date plus 99).