What is 90 days from today?

Ninety days is twelve weeks and six days, so it lands on the same weekday as today plus six. From March 15, 2026 for example, 90 days later is June 13, 2026, a Saturday. Because ninety days spans three months of differing lengths it is not the same as "three months" — three months from the 31st clamps to the end of a short month, while ninety days keeps counting. The lookup chart on this page gives the exact date for the common offsets from today: 30, 45, 60, 90, 120, 180, and 365 days, forwards and backwards.

Why does adding one month to January 31 give February 28?

Because the day-of-month is clamped to the length of the target month. February has only 28 days in 2026, so "the 31st of February" does not exist and the result becomes February 28, 2026 — the last day of the month — rather than overflowing to March 2 or March 3. In a leap year the same start date lands on February 29, 2024. Every spreadsheet, bank, and payroll system uses this convention, and it is why month arithmetic is not reversible: Jan 31 + 1 month = Feb 28, but Feb 28 − 1 month = Jan 28.

How many days are in a year, and which years are leap years?

A common year has 365 days; a leap year has 366. A year is a leap year if it is divisible by 4 — except century years, which must also be divisible by 400. That means 2000 was a leap year but 1900 was not, and 2100, 2200, and 2300 will not be either. The exception exists because the Earth's orbit takes about 365.2422 days, so adding a day every four years slightly overcorrects; dropping three leap days every four centuries brings the calendar back into line to within about one day in 3,000 years.

What is the difference between calendar days and business days?

Calendar days count every day on the calendar. Business days — also called working days or weekdays — count only Monday to Friday, and usually exclude public holidays as well. A two-week span is 14 calendar days but only 10 business days, and fewer still if a holiday falls inside it. Contracts, shipping estimates, and legal deadlines almost always mean business days, so "5 days to process" typically means a full working week, and a Friday request is not due back until the following Friday.

Does daylight saving time affect a date calculation?

It should not, and it does not here. Two days a year are 23 or 25 hours long in clock terms, so a calculator that works in local timestamps and divides by 24 hours can drop or gain a whole day across a spring-forward or fall-back boundary. This calculator does all its arithmetic on plain calendar dates pinned to UTC, where every day is exactly 24 hours, so the answer is the same regardless of your timezone or whether a clock change falls inside the span.

How do I work out what day of the week a date falls on?

Any date advances one weekday per common year and two per leap year, so if your birthday is a Tuesday this year it is a Wednesday next year, unless a February 29 intervenes and it jumps to Thursday. Over a full 400-year Gregorian cycle there are exactly 146,097 days, which is divisible by 7 — so the whole calendar repeats every 400 years, and the 13th of the month falls on a Friday slightly more often than on any other weekday. This calculator names the weekday for every date it returns.

How many weeks are in a month?

Between 4 and 5. A 28-day February is exactly four weeks; a 31-day month is four weeks and three days. Averaged over a year a month is about 4.35 weeks, which is why a monthly figure is never a clean multiple of a weekly one, and why payroll systems that pay every two weeks produce three paychecks in two months of the year rather than two. Converting between months and days always needs the actual calendar, not a 30-day approximation.

Time

Date Calculator

Days between two dates = later date − earlier date; to add a duration you move the whole months first, clamped to the end of the target month, then the weeks and days. This calculator does both directions, plus business days only — Monday to Friday with the eleven US federal holidays taken out — and names the weekday every answer lands on.

What do you need?

Start date

The date you are counting from

Pick a date

Duration to apply

Years and months are applied first, then weeks and days.

Years

Months

Weeks

Days

Pick a start date and a duration to see the resulting date and its weekday.

Try a scenario

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The basics

What a date calculator actually does

Two questions hide behind the same tool, and they run in opposite directions. One takes a date and a length of time and asks where you land. The other takes two dates and asks how far apart they are. Everything else on this page — business days, weekday names, month breakdowns — is a refinement of one of those two.

date + duration → date

Adding or subtracting a duration

You know a starting point and a length of time — a notice period, a warranty, a due date — and you want the calendar date at the other end. Days and weeks are exact counts; months and years follow the calendar, so their length depends on where you start.

2026-03-15 + 90 days → 2026-06-13 (Saturday)

date − date → duration

Measuring the gap between two dates

You know both endpoints and want the distance. The same gap can be told in days, weeks, whole months, or years-months-days, and the four answers are not interchangeable, because real months are between twenty-eight and thirty-one days long.

2026-03-01 → 2026-09-15 = 198 days = 6 months, 14 days

Method

How to do it by hand

Four steps cover both directions. The order in step two is the part people get wrong.

1
Write both dates as year, month, day
Strip the time of day entirely — a date calculation is calendar arithmetic, not clock arithmetic. Writing them as YYYY-MM-DD also removes the US/European day-month ambiguity that causes most real-world errors.
2026-03-15 → 2026, month 3, day 15
2
Apply years and months first, clamping the day
Convert years to months, add them to the starting month, and carry into the year. If the resulting day-of-month does not exist in the target month, clamp it to that month’s last day. Doing this before the days is what keeps the result reproducible.
2026-01-31 + 1 month → 2026-02-28, not March 3
3
Then add the weeks and days, rolling over month ends
Weeks are just seven days. Add the total to the day-of-month and roll forward month by month, subtracting each month’s real length as you cross it — thirty-one for January, twenty-eight or twenty-nine for February, and so on.
2026-03-15 + 90 d: 16 + 30 + 31 + 13 → 2026-06-13
4
For a gap, subtract — then say which count you used
Convert both dates to a day number and subtract for the exclusive count, then add one if both endpoints should be counted. State the convention alongside the number; an unlabelled day count is the single biggest source of disagreement about dates.
198 exclusive · 199 inclusive

Counting rules

Inclusive or exclusive — the question that changes the answer

Almost every “your calculator is wrong” complaint about dates is really a disagreement about whether the endpoints count. There is no universally correct answer, only a correct answer for the context — so here is exactly what each mode of this calculator does.

end − start

Exclusive: the start date is day zero

The default for a duration. It answers “how much time has passed”, which is why ages, notice periods, and interest accrual all use it. Two consecutive days are one day apart, and a date is zero days from itself.

Jul 4 → Jul 5 = 1 day · 2026-03-01 → 2026-09-15 = 198 days

end − start + 1

Inclusive: both endpoints are counted

The default for a quantity of days. It answers “how many days are involved”, which is what a holiday allowance, a jury summons, a billing period, or a working-day count means. A single date is one day, not zero.

Jul 4 → Jul 5 = 2 days · 2026-03-01 → 2026-09-15 = 199 days

Mode on this page	Counting used	Jul 4 → Jul 5 gives	Why
Add / subtract	Exclusive	1 day moved	A duration applied to a date — the start is the origin, not an item.
Days between (headline)	Exclusive	1 day	Matches the way elapsed time, age, and interest are measured.
Days between (badge)	Inclusive	2 days	Shown alongside so you never have to guess which one you are reading.
Business days between	Inclusive	2 working days	Payroll and HR count the working days in a period, endpoints included.
N business days from a date	Exclusive	Jul 5 is +1	A deadline counts forward from the day after the trigger event.

The two counts always differ by exactly one. If a number here disagrees with another tool, check this first — it is the explanation nine times out of ten.

Worked examples

Four calculations, worked through

One example per mode, with the intermediate steps left in. Switch tabs to see how the working changes when months are involved rather than days.

Givenstart = 2026-03-15 (Sunday), add 90 days

March has 31 days, so 16 days finish the month → 2026-03-31

April 30 + May 31 = 61 more days → 2026-05-31

13 days remain of the 90 → 2026-06-13

Check: 2026-06-13 − 2026-03-15 = 90 days

Saturday, June 13, 2026

= 12 weeks, 6 days after the start

Day offsets never need clamping — every calendar day is exactly 24 hours long in this arithmetic, so 90 days is 90 days whether or not a daylight-saving change or a leap day falls inside the span.

Month arithmetic

Why January 31 plus one month is February 28

Months are not a fixed length, so “plus one month” cannot be done by adding a number of days. The rule everyone uses is: keep the day-of-month if it exists in the target month, and otherwise clamp to that month’s last day. Below is every month-end in 2026 and where a one-month jump lands.

clamp, never overflow

The clamp keeps you inside the month

A naive implementation that just increments the month number produces a date in the month after next. Clamping is what every spreadsheet, payroll system, and loan schedule does instead, and it is what makes “the last day of next month” work.

2026-01-31 + 1 month → 2026-02-28 · leap year → 2024-02-29

not reversible

Month arithmetic loses information

Once a date has been clamped, the original day-of-month is gone, so adding a month and subtracting it again need not return you to the start. If a contract must round-trip exactly, express the term in days rather than months.

Jan 31 + 1 mo = Feb 28, but Feb 28 − 1 mo = Jan 28

Month (2026)	Days	Last day	Plus one month	Clamped?
January	31	2026-01-31	2026-02-28	yes
February	28	2026-02-28	2026-03-28	—
March	31	2026-03-31	2026-04-30	yes
April	30	2026-04-30	2026-05-30	—
May	31	2026-05-31	2026-06-30	yes
June	30	2026-06-30	2026-07-30	—
July	31	2026-07-31	2026-08-31	—
August	31	2026-08-31	2026-09-30	yes
September	30	2026-09-30	2026-10-30	—
October	31	2026-10-31	2026-11-30	yes
November	30	2026-11-30	2026-12-30	—
December	31	2026-12-31	2027-01-31	—

Leap years

The leap-year rule, in full

“Every four years” is only two thirds of the rule, and the missing third is what makes long date spans come out right. A year is a leap year if it divides by four — except century years, which must also divide by four hundred.

year % 4 === 0

The base rule

Nearly all leap years qualify here. The solar year is close to three hundred and sixty-five and a quarter days, so an extra day every fourth year absorbs the accumulated quarters.

except % 100

The century exception

A quarter day a year is a slight overcorrection, so century years are demoted back to common years. This is the step that catches out anyone who only remembers the base rule.

unless % 400

The exception's exception

Skipping three leap days every four centuries slightly undercorrects, so one century in four keeps its leap day. That restores the calendar to within about a day in three thousand years.

Century year	÷ 4	÷ 100	÷ 400	Leap year?	Days in February
1600	yes	yes	yes	Leap	29
1700	yes	yes	no	Common	28
1800	yes	yes	no	Common	28
1900	yes	yes	no	Common	28
2000	yes	yes	yes	Leap	29
2100	yes	yes	no	Common	28
2200	yes	yes	no	Common	28
2300	yes	yes	no	Common	28
2400	yes	yes	yes	Leap	29

The next eight leap years: 2024 · 2028 · 2032 · 2036 · 2040 · 2044 · 2048 · 2052. A full Gregorian cycle is 400 years and exactly 146,097 days — a whole number of weeks, which is why the calendar repeats precisely every four centuries.

Working days

Counting business days only

Switch the calculator to Business days and it drops Saturdays and Sundays, then removes any of the eleven US federal holidays that land on a weekday. 2026 has 261 weekdays and 250 working days once the 11 observed federal holidays are removed. For a fully configurable working week and a per-month breakdown, use the dedicated business days calculator.

2026 federal holiday	Rule	Date	Falls on	Observed
New Year's Day	January 1	Jan 1, 2026	Thursday	same day
Martin Luther King Jr. Day	3rd Monday in January	Jan 19, 2026	Monday	same day
Washington's Birthday (Presidents' Day)	3rd Monday in February	Feb 16, 2026	Monday	same day
Memorial Day	Last Monday in May	May 25, 2026	Monday	same day
Juneteenth National Independence Day	June 19	Jun 19, 2026	Friday	same day
Independence Day	July 4	Jul 4, 2026	Saturday	Jul 3, 2026
Labor Day	1st Monday in September	Sep 7, 2026	Monday	same day
Columbus Day (Indigenous Peoples' Day)	2nd Monday in October	Oct 12, 2026	Monday	same day
Veterans Day	November 11	Nov 11, 2026	Wednesday	same day
Thanksgiving Day	4th Thursday in November	Nov 26, 2026	Thursday	same day
Christmas Day	December 25	Dec 25, 2026	Friday	same day

Rules current as of 2026 (5 U.S.C. § 6103, including Juneteenth, added 2021). A fixed-date holiday falling on a Saturday is observed the Friday before; on a Sunday, the Monday after.

Quick chart

Common offsets from today

The dates people look up most: 30, 60, 90, and 180 days from today, plus the same in weeks and months, forwards and backwards, with the weekday named. Every row is computed from your own calendar date.

Counted from today’s date in your own calendar.

Offset	Equivalent	That many days from today	Weekday	That many days ago
1 day	1 day	—	—	—
2 days	2 days	—	—	—
3 days	3 days	—	—	—
5 days	5 days	—	—	—
7 days	1 week	—	—	—
10 days	1 week, 3 days	—	—	—
14 days	2 weeks	—	—	—
15 days	2 weeks, 1 day	—	—	—
21 days	3 weeks	—	—	—
30 days	4 weeks, 2 days	—	—	—
45 days	6 weeks, 3 days	—	—	—
60 days	8 weeks, 4 days	—	—	—
90 days	12 weeks, 6 days	—	—	—
100 days	14 weeks, 2 days	—	—	—
120 days	17 weeks, 1 day	—	—	—
180 days	25 weeks, 5 days	—	—	—
200 days	28 weeks, 4 days	—	—	—
270 days	38 weeks, 4 days	—	—	—
365 days	52 weeks, 1 day	—	—	—

Day and week offsets are exact: every row is a whole number of 24-hour calendar days from today, weekends and holidays included.

Weekdays

What day of the week will it be?

The weekday of a date is not random — it drifts by a predictable amount each year, which is why you can often work it out in your head without a calendar.

365 = 52 × 7 + 1

A date advances one weekday a year

A common year is fifty-two weeks and one day, so every date lands one weekday later than it did last year. That single leftover day is the whole reason calendars cannot simply be reprinted each year.

2026-03-15 is a Sunday; a year later it is a Monday

366 = 52 × 7 + 2

Two weekdays across a leap day

A leap year has two leftover days, so any date falling after February 29 skips forward two weekdays rather than one. Dates in January and February still move by one, because the extra day has not happened yet.

Dates after Feb 29 jump two weekdays instead of one

146,097 days

The calendar repeats every 400 years

A full Gregorian cycle contains a whole number of weeks, so the calendar for 2026 is identical to the one for 2426. Within that cycle the thirteenth of the month falls on a Friday slightly more often than any other weekday.

weekday of the answer

Why the weekday matters more than the date

A deadline that lands on a Saturday usually is not really a deadline. Any date this calculator returns is labelled with its weekday, so you can see immediately whether the answer needs rolling to the next working day.

2026-06-13 is a Saturday

Glossary

The terms a date calculation uses

Five words that mean something specific here, and that ambiguity in any of them will change the answer you get.

Calendar day: One day on the calendar, regardless of weekday or holiday. Always exactly 24 hours in this calculator, because the arithmetic runs on plain dates in UTC rather than on local clock time.
Business day / working day: Monday to Friday, excluding public holidays. The default working week here is Mon-Fri with the eleven US federal holidays optionally removed; contracts sometimes define their own set, so check the wording.
Inclusive count: Both the first and last dates are counted as days. A period that runs from the 1st to the 5th is five days inclusive.
Exclusive count: Only the time between the dates is counted; the start date is day zero. That same period from the 1st to the 5th is four days exclusive.
Clamping: Pulling an impossible day-of-month back to the last real day of the target month when months are added or subtracted, so January 31 plus a month is the end of February rather than a date in March.
Observed date: The day a holiday is actually taken when the statutory date falls at the weekend — the Friday before for a Saturday, the Monday after for a Sunday. Business-day counting uses the observed date, not the statutory one.

Where it shows up

When you need this

Date arithmetic is invisible until it is wrong, and then it is expensive. These are the places it matters most.

notice and deadlines

Contracts and legal filings

Notice periods, cooling-off windows, and filing deadlines are counted in days from a trigger event — and almost always in business days, with the convention stated in the clause itself.

due dates

Medical and project planning

Gestational dates, follow-up appointments, and project milestones are all a start date plus a fixed number of days or weeks, and all care whether the answer falls at a weekend.

billing periods

Finance and payroll

Interest accrues per day, subscriptions renew per month, and payroll runs per fortnight — three different clocks that only line up if the day counts are done against the real calendar.

Worth knowing

Rules worth remembering

Six things that decide whether two people counting the same span agree on the number.

say which count you used

An unlabelled day count is ambiguous

Inclusive and exclusive counts always differ by one. Writing “14 days inclusive” rather than “14 days” removes an entire category of dispute.

a month is not 30 days

Never convert months to days with a multiplier

Real months run from twenty-eight to thirty-one days. Any conversion that assumes thirty drifts by up to three days a month and by nearly a week over a year.

6 months here is 198 days, not 180

months first, then days

Order of operations changes the answer

Adding one month then one day is not always the same as adding one day then one month, because the clamp happens in between. This calculator always applies years and months first.

write dates as YYYY-MM-DD

Ambiguous formats cause real errors

03/04 is March fourth in the US and the third of April almost everywhere else. The ISO order sorts correctly as text and cannot be misread.

UTC, not local time

Timezones can move a date by a day

A calculation done on local timestamps can gain or lose a day across a daylight-saving change or a timezone boundary. Plain calendar dates avoid the problem entirely.

check the weekday

A deadline on a Saturday is not a deadline

Most rules roll a due date that lands at the weekend to the next working day. Always look at the weekday of the answer before treating it as final.

2026-06-13 → Saturday

Common questions

Subtract the earlier date from the later one and count the whole calendar days in between. That standard, exclusive count treats the start date as day zero, so July 4 to July 5 is one day. If you need both endpoints counted — the way a hotel counts nights differently from days, or a contract counts "days inclusive" — add one to the result. The two answers always differ by exactly one, and stating which convention you used removes almost every argument about whether a date calculator is right.

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