Date Calculator

What Is a Date Calculator?

A date calculator is a tool that performs addition and subtraction on calendar dates, where the units are not uniform. Ordinary arithmetic assumes every unit is the same size; the calendar does not, and that single difference is the whole reason this page exists.

The Two Questions It Answers

Almost every real date problem is one of two shapes. The first is a duration: two dates are already known and the gap between them is what you need — a lease term, a project timeline, a notice period, a countdown to an event, the days an invoice has been outstanding. The second is an offset: one date is known, and a date a stated distance away from it is what you need — a return window, a warranty expiry, a 90-day probation end, a follow-up appointment.

The mode selector at the top of the calculator picks between them, and each mode reads only its own fields. In days-between mode the Start Date and End Date boxes are used and the direction, amount and unit boxes are ignored. In add-or-subtract mode the End Date box is ignored and the direction, amount and unit boxes take over. Nothing needs clearing when you switch.

Why the Real Calendar Beats a 30-Day Month

The convenient shortcut — treat every month as 30 days, every year as 360 — is still used in some bond markets, and it is wrong by up to two days per quarter for anything measured on an actual calendar: three consecutive months run 89 days at the shortest and 92 at the longest, landing on the model’s 90 only when the months happen to line up — about one start date in seven. February is 28 days, seven months are 31, and 97 years in every 400 carry a leap day — one year in four, less three of the century years. Those irregularities do not cancel out over short spans; they accumulate in whichever direction your particular dates happen to fall.

Everything on this page runs on real month lengths. February 1 to March 1 returns 28 days in 2026 and 29 days in 2024, from the same two calendar positions. A 30-day model returns 30 for both and is wrong twice.

Every Answer Names Its Weekday

Both modes print the weekday alongside every date, which is not decoration. A deadline that lands on a Sunday is a deadline that effectively lands on the following Monday for anything involving an office, a bank or a courier, and you cannot see that from the number 90. Adding 90 days to Saturday, August 15, 2026 gives Friday, November 13, 2026 — workable. Adding 120 gives Sunday, December 13, 2026 — not.

Measuring from a date of birth instead? The Age Calculator.

How Do You Calculate the Days Between Two Dates?

Subtract the earlier date from the later one and read the answer in whole days. The difficulty is never the subtraction — it is that the two dates are written in a mixed-radix system where the middle digit changes size as you move along it.

The Date Formula, Written Out

The total-days figure is a single subtraction once both dates are converted to a day count from a fixed origin. The other two lines are re-expressions of that same total.

Total days   = Later date − Earlier date   (end date not counted)

Weeks line   = Total days ÷ 7, whole part
               remainder shown as leftover days

Years / months / days
             = years difference, then months difference,
               then days difference — borrowing the real
               length of the month before the end date
               whenever the day difference is negative

The borrow step is where hand calculation usually fails. Going from March 2 to June 15 the day figure is positive and nothing is borrowed; going from March 20 to June 15 it is negative, so one month is given up and the 31 days of May are added back.

Step by Step With This Calculator

• Leave the mode set to “Days between dates”, or switch back to it.
• Set the Start Date. It arrives pre-filled, but with the date this page was last built rather than the date on your own clock, so set it to the day you are counting from.
• Set the End Date. It opens on December 31, 2026, which makes the default state a days-left-in-the-year counter.
• Read the big number — that is the total in days. Order does not matter: the calculator sorts the two dates itself, so an end date earlier than the start date returns the same positive figure rather than a negative one.
• Add 1 to the total if your situation counts both endpoints, such as billing a rental for the day of pickup and the day of return.

Worked Example: January 1 to July 10, 2026

Entering January 1, 2026 as the start and July 10, 2026 as the end returns 190 days on the first line, then 0 years, 6 months, 9 days, then 27 weeks and 1 day, then a restatement of both dates with their weekdays — Thursday to Friday.

Duration mode, January 1, 2026 → July 10, 2026

190 days = 27 weeks and 1 day = 0 years, 6 months, 9 days

Check it by hand and the three lines agree. January through June is 31 + 28 + 31 + 30 + 31 + 30 = 181 days from January 1 to July 1, plus the 9 days from July 1 to July 10, giving 190. Divide by 7 and you get 27 with 1 left over. The years-months-days line reads January 1 plus six months as July 1, then nine more days to July 10 — the one line of the three that is a description of the route rather than a measurement of the distance.

Why the End Date Is Not Counted

Counting the gap rather than the days touched is the convention behind deadlines, notice periods, rescission windows and daily interest, and it is what makes a Friday-to-Friday week come out at 7 rather than 8. Monday, August 10 to Friday, August 14, 2026 returns 4 days, because four midnights separate them.

The alternative — inclusive counting, where both endpoints are days on the tally — is what you want for hotel nights against days away, for a conference that runs the 3rd through the 7th, or for a jury summons. The fix is arithmetic you can do in your head: add 1. January 1 to December 31, 2026 returns 364; add 1 and you have the 365 days of the year.

How to Add or Subtract Days, Weeks, Months or Years

Switch the mode selector to “Add / subtract from a date”, set a start date, pick Add or Subtract, type a whole amount and choose the unit. The result is a single date with its weekday. The four units do not all behave the same way, and the difference matters most at the end of a month.

Days and Weeks Are Exact Arithmetic

Days and weeks are fixed-size units, so these two are pure counting and always reversible. A week is 7 days with no exceptions anywhere in the calendar, which is why adding weeks always returns the same weekday you started on: 2 weeks after Saturday, August 15, 2026 is Saturday, August 29, 2026, and 8 weeks after it is Saturday, October 10, 2026.

Adding days moves the weekday by the remainder of the amount divided by 7. Thirty days is 4 weeks and 2 days, so 30 days after that same Saturday is a Monday — September 14, 2026. Ninety days is 12 weeks and 6 days, so it lands six places further round the week, on Friday, November 13, 2026.

Months and Years Clamp to the Last Day

Adding a month means landing on the same day number in the next month, and one month in four has no 31st, while February has no 30th or 31st at all. The rule this calculator applies is the standard one: keep the day number if it exists in the target month, otherwise fall back to that month's last day.

• January 31, 2026 + 1 month → Saturday, February 28, 2026
• January 31, 2024 + 1 month → Thursday, February 29, 2024, because 2024 is a leap year
• May 31, 2026 + 1 month → Tuesday, June 30, 2026
• August 31, 2026 + 6 months → Sunday, February 28, 2027
• February 29, 2024 + 1 year → Friday, February 28, 2025
• March 31, 2026 − 1 month → Saturday, February 28, 2026

This is the same behavior as the EDATE function in spreadsheets and as most monthly billing systems, so a subscription that starts on the 31st bills on the 28th, 29th or 30th in the short months and returns to the 31st afterwards. Note that clamping destroys information: January 29, January 30 and January 31, 2026 all land on February 28, 2026, so the operation cannot be run backwards to recover which one you started from.

If what you actually want is “the last day of each month”, do not clamp — add the months to the 1st of the following month and subtract one day. From August 15, 2026, adding 1 month gives Tuesday, September 15, 2026; from September 1 plus 1 month minus 1 day you get September 30 instead.

Worked Examples From August 15, 2026

Every figure below comes from the calculator with Saturday, August 15, 2026 in the Start Date box.

Subtracting is the same machinery with the sign flipped, so there is no need to type a minus in the amount box — set the direction instead. A decimal typed there is rounded to the nearest whole unit before the shift, and zero or a negative returns the amber notice rather than a date.

Date Chart: Month Lengths and Common Intervals

Two reference tables. The first is the raw material of every calculation on this page; the second shows where the intervals people actually type — 30, 60, 90, 180 — end up.

Days in Each Month

Seven months have 31 days, four have 30 and one has 28 or 29. The lengths also cluster: July and August are both 31, which is why a span running across that pair is two days longer than the same nominal span across September and November.

Where Common Intervals Land

Measured from Saturday, August 15, 2026, in add mode with the unit set to days.

Only the multiples of 7 keep the original weekday. Everything else drifts by the remainder, which is what makes a 45-day or 100-day deadline land somewhere you would not have guessed.

Is 90 Days the Same as Three Months?

No, and the gap widens as the interval grows. Contracts, refund policies and probation periods use both phrasings as if they were interchangeable, and they are not — the same start date produces different end dates depending on which unit you pick.

The three months from August 15 to November 15, 2026 cover 92 days because August, September and October run 31, 30 and 31. Start the same three months at February 15 and they cover 89 days; start them at December 1 and they cover 90. If a document says “90 days”, count days. If it says “three months”, count months. Whoever drafted it chose one.

How to Read Your Result

The two modes print different result panels, and each line answers a different question. Reading the wrong line is the most common way to get a correct calculation and still act on the wrong number.

The Four Lines in Days-Between Mode

• The headline total in days. This is the measurement — the one figure that is exact, unambiguous and safe to bill, accrue or contest against.
• The years, months and days breakdown. A human-readable route rather than a measurement; useful for describing a span, not for arithmetic on it.
• The weeks line — whole weeks plus leftover days. This is the line to start from when you need business days, because a full week always contains exactly 5 weekdays.
• The restated span, with both dates spelled out and their weekdays named, plus the reminder that the end date is not counted. Use it to confirm the calculator read your dates the way you meant them.

In add-or-subtract mode the panel is shorter: the resulting date with its weekday on the first line, a restatement of what was done to what on the second, and a note of which unit was counted on the third.

Which Line to Use for What

Use the total in days for anything that accrues per day — interest, per-diem rates, prorated rent, late fees, visa stays measured in days. Use the weeks line for staffing and scheduling, where the working week is the natural unit. Use the years-months-days line for describing a tenure or a term to a person, and for nothing that gets multiplied.

The reason for that last restriction: 89, 90 and 92 days can all report as 3 months, 0 days, because the calendar months they span are different lengths. February 1 to May 1, 2026 is 89 days; January 15 to April 15, 2026 is 90; June 1 to September 1, 2026 is 92. All three read 3 months on the breakdown line, and only the totals line separates them.

When It Says Check Your Inputs

Three states produce the amber notice instead of a result, and each names the field at fault: an unreadable or empty start date, an unreadable or empty end date, and an amount that is not a positive number.

• The amount box takes positive whole numbers only. Zero is rejected, and so is a negative — to go backwards, leave the amount positive and set the direction to Subtract.
• Decimals are rounded to the nearest whole unit before anything happens, so an amount of 2.6 days is treated as 3 days and returns Tuesday, August 18, 2026 from an August 15 start. There is no such thing as half a day here.
• Both date boxes must hold a real calendar date. Setting the same date in both is legal and returns 0 days, which is the correct answer rather than an error.

Leap Years and the Weekday Drift

Two calendar facts explain nearly every surprising result on this page: a year is not a whole number of days, and it is not a whole number of weeks either.

The Rule, as NIST States It

A year is a leap year if it is divisible by 4, except that century years are leap years only when divisible by 400. NIST's time and frequency glossary spells out the consequences by name: 1700, 1800 and 1900 were not leap years and 2100 will not be one, while 1600 and 2000 were. The correction exists because the actual year is about 365.2422 days, so adding a day every four years overshoots by roughly three days per four centuries.

The effect is directly visible in the calculator. February 1 to March 1 returns 29 days in 2024 and in 2000, and 28 days in 2026 and in 2100 — four different centuries, two different answers, one rule.

• A common year is 365 days — 52 weeks and 1 day.
• A leap year is 366 days — 52 weeks and 2 days.
• A full 400-year Gregorian cycle is 146,097 days. January 1, 1600 to January 1, 2000 returns exactly that, and it divides evenly into 20,871 weeks.

Why a Fixed Date Moves Through the Week

Because 365 is 52 weeks plus a remainder of 1, any fixed calendar date advances one weekday each year, and two after a leap year. January 1 falls on a Wednesday in 2025, a Thursday in 2026, a Friday in 2027 and a Saturday in 2028 — then skips Sunday and lands on Monday in 2029, because 2028 is a leap year.

Since 146,097 days divides exactly by 7, the entire pattern repeats every 400 years: the Gregorian calendar for 2026 is identical to the calendar for 1626 and for 2426. It also means anniversaries drift. August 15, 2026 is a Saturday; August 15, 2027 is a Sunday; August 15, 2028 is a Tuesday, having skipped Monday over the February 29 in between.

Calendar Days vs Business Days

This tool counts calendar days — every day, weekends and holidays included. That is the right unit for leases, interest, visa stays and most statutory deadlines, and the wrong one for shipping estimates, court filings and payroll cut-offs.

Two Definitions That Are Both Official

There is no single legal meaning of “business day”, which is why counting them requires knowing whose rules apply. Regulation Z, the Consumer Financial Protection Bureau's implementation of the Truth in Lending Act, carries two definitions in the same paragraph: the general one is any day a creditor's offices are open for substantially all its business functions, while for rescission and certain mortgage disclosures it means all calendar days except Sundays and the federal holidays listed in 5 U.S.C. 6103(a).

Read that second definition again — under it, Saturday is a business day. A contract that says “three business days” can mean three different end dates depending on which rulebook it sits under, and no calculator can resolve that for you. Establish the definition first, then count.

Converting the Weeks Line Into Weekdays

The weeks line makes a Monday-to-Friday estimate straightforward, because every complete week holds exactly 5 weekdays regardless of where it starts. Take the whole weeks, multiply by 5, then walk the leftover days forward from the start date and count only the ones that are not a Saturday or a Sunday.

Weekdays ≈ (whole weeks × 5) + weekdays among the leftover days

August 15 → November 13, 2026
  = 90 days = 12 weeks and 6 days
  = (12 × 5) + 4
  = 64 weekdays, before holidays are removed

The leftover six days run from Saturday, November 7 through Thursday, November 12, of which four are weekdays. Then subtract whichever holidays your calendar observes — Veterans Day, Wednesday, November 11, sits inside that span for anyone following the federal list, bringing 64 down to 63.

US Federal Holidays in 2026

The eleven days named in 5 U.S.C. 6103(a), with the weekday each falls on in 2026. Subsection (b) moves a Saturday holiday to the preceding Friday for federal employees on a Monday-to-Friday week.

Ten of the eleven land on a weekday in 2026 outright, and the eleventh is observed on one, so all eleven cost a working day. State holidays, bank holidays outside the US and company shutdowns are additional and are not on this list.

Limits: When This Calculator Does Not Apply

Where the answer stops being reliable, and what to use instead.

• It does not count business days, and cannot. Weekend and holiday rules vary by country, state and industry, and the two Regulation Z definitions above disagree with each other. Use the weeks line to estimate, then apply your own holiday calendar.
• It works in whole days, not hours. Both dates are read at midnight, and the difference is rounded to whole days — which is exactly why daylight saving time never disturbs a count. March 1 to March 15, 2026 returns 14 days across the spring-forward on March 8, and October 25 to November 8 returns 14 across the fall-back on November 1.
• It is not a time zone converter. Both boxes hold plain calendar dates with no location attached, so a deadline expressed in another zone has to be translated to a local date before you type it.
• For spans that start on the 29th, 30th or 31st, trust the total-days line over the years-months-days line. The breakdown borrows the length of the month before your end date, which is a different convention from the clamping rule that add-mode uses, so the two do not always round-trip. The day total is unaffected and is always exact.
• It applies the Gregorian calendar to every date, including those before Gregorian dates were in use. Britain and its colonies switched in September 1752 and Russia in 1918, so a historical date taken from a document written before the local changeover will not line up with what this page reports.
• Leap seconds do not appear. They are added to Coordinated Universal Time at the end of June or December to keep clocks aligned with Earth's rotation, and they change the length of a day by one second — never the date on the calendar.
• It handles one span or one shift at a time. Recurring schedules, payroll periods, amortization dates and working-day offsets need a tool built around a repeat rule, not a single subtraction.