Date Calculator
Work out how many days fall between two dates, or add and subtract days from any starting date.
Calculation results
Frequently used days
Calculation results
Base date
How many days are between two dates?
Pick a start date and an end date above and the day count appears instantly — no button, nothing sent to a server. I built this date calculator because I kept doing the same awkward finger-counting on a wall calendar to work out deadlines, and I always lost track somewhere around the end of the month.
The difference tab gives you the gap in plain days first, and then, once the span is long enough, it also restates it in weeks, months, and years so a number like 400 days reads as “1 year, 1 month, 5 days” too. If your end date falls before your start date, the result simply shows as negative.
What does “include start date” actually change?
It adds one day to the total, because it counts the start date itself as day one. By default I exclude the start date, so the result is the pure gap between the two days — the way you’d normally answer “how many days until my trip?”
Switch to “include start date” when both ends of the range count. Booking a meeting room from Monday to Friday, counting vacation days you’ll be away, or billing a stay where the first day is chargeable — in all of those, the first day is part of what you’re counting, so the +1 is exactly right.
How do I add or subtract days from a date?
Open the second tab, set a base date, choose addition or subtraction, and type the number of days — the resulting date updates as you go. Instead of typing, you can tap one of the quick chips (1, 7, 14, 30, 90, 180, and 365 days) for the spans people ask for most.
The result is written out in full, with the weekday included, so you can see at a glance that 30 days from today lands on a Saturday, not just a date. That weekday is usually the part I actually care about.
How does it handle leap years and date standards?
This calculator follows the Gregorian calendar — the civil calendar introduced in 1582 and used almost everywhere today. Its mean year is exactly 365.2425 days, the figure you get by spreading 146,097 days across a 400-year cycle that holds 97 leap years (146,097 ÷ 400). That precision is why long spans, where forgetting even one leap day throws the count off, come out cleaner than when I tally them on a wall calendar.
The leap-year rule itself is short: a year is a leap year if it is divisible by 4, except that years divisible by 100 are not — unless they are also divisible by 400. So 2000 was a leap year, but 1900 and 2100 are not. You can check the rule against the U.S. Naval Observatory’s leap years explainer.
Dates are handled in the international YYYY-MM-DD form defined by ISO 8601. RFC 3339, the standard for Internet timestamps, sums up how that format relates to the calendar:
“This document defines a date and time format for use in Internet protocols that is a profile of the ISO 8601 standard for representation of dates and times using the Gregorian calendar.” — RFC 3339, Date and Time on the Internet: Timestamps
Week numbers follow the same ISO 8601 rule: a week starts on Monday, and week 1 of a year is the week containing that year’s first Thursday — equivalently, the week that holds 4 January. That is why, for a few days around New Year, the calendar year and the week-numbering year can differ by one; the ISO week date article lays out the edge cases.
Here are the key figures at a glance:
| Item | Value | Standard |
|---|---|---|
| Mean Gregorian year | 365.2425 days | Gregorian calendar |
| Leap-year cycle | 97 leaps per 400 years (÷4, not ÷100, but ÷400) | Gregorian calendar |
| One day | 86,400 seconds | POSIX / Unix time |
| Date format | YYYY-MM-DD | ISO 8601 |
| Week 1 definition | week of the first Thursday (holds 4 Jan, starts Monday) | ISO 8601 |
Under the hood it uses the browser’s JavaScript Date object, which tracks time as milliseconds since midnight UTC on 1 January 1970, treating each day as exactly 86,400 seconds (24 × 60 × 60), leap seconds aside. MDN’s Date reference documents exactly how that works.
When is a date calculator genuinely useful?
The moment a date has to be exact rather than roughly remembered. A few cases I reach for it:
- D-day countdowns to an exam, a launch, a due date, or a trip.
- Deadlines measured in days — a 90-day notice period, a 14-day return window, a 180-day visa limit.
- Contracts and warranties that start on one date and expire a set number of days later.
- Anniversaries and milestones, like your 100th or 1,000th day.
Everything runs in your browser, so even sensitive dates from a contract never leave your device.