Day Calculator
Pick a date, choose how much to add or subtract, and get the exact resulting date — with its weekday, the day of the year it falls on, and how far it is from today.
What this day calculator does
Enter a starting date, an amount and a unit — days, weeks, months or years — and the calculator lands on the exact resulting date. It also tells you the weekday, which day of the year it is, its ISO week number and how many days separate it from today.
Month arithmetic follows the calendar rather than a fixed 30-day convention: adding one month to January 31 gives February 28 (or 29 in a leap year), never March 2 or 3. Adding one year to February 29 lands on February 28 the following year.
Weekday arithmetic, explained
Weekdays repeat in a 7-day cycle, so adding 7, 14 or 21 days always lands on the same weekday. Adding 100 days shifts the weekday by 2 (100 mod 7 = 2) — a trick you can use to double-check any result by hand.
The day-of-year counter helps with deadlines phrased as "within 90 days" or annual milestones like the 100th day of school. The ISO week number matches the numbering used across Europe and in most business planning tools.
Common uses
Deadlines and follow-ups. Court dates, payment terms, return windows and project milestones are usually set as N days from a trigger date.
Renewals. Subscriptions and memberships renew on a monthly or yearly cadence — the month and year modes keep the same day of month when it exists.
Planning. Countdown to a trip, an exam or a launch: flip to subtract, or just read the distance-from-today line in the result.
Frequently asked questions
What day is 100 days from today?
Open the page and type 100 into the amount box with the unit set to days — the result shows the exact date and its weekday. The calculator uses your device's clock, so "today" is always correct.
Why doesn't January 31 plus one month equal March 3?
Because a calendar month is not 30 days. The calculator clamps to the last day of the target month: January 31 + 1 month = February 28 (or 29 in a leap year). Adding the same date back would be wrong for most deadlines.
Does the result include the start date?
No. Adding 1 day to January 1 gives January 2 — the standard convention for deadlines. If you need both endpoints counted, use the days-between calculator which shows an inclusive figure.
Is this accurate across leap years?
Yes. The underlying date arithmetic uses the Gregorian calendar rules: every 4th year is a leap year except century years not divisible by 400.
Can I share a specific calculation?
Yes — every result updates the page URL with the date, amount, unit and operation, so you can bookmark or send the exact calculation.
Does it work offline?
Yes. Everything runs in JavaScript in your browser — no server round-trips, so it also keeps working once the page has loaded without a connection.