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Calculate the duration between two dates

This date duration calculator tells you exactly how much time separates two dates — in years, months, weeks and days, and as running totals in days, weeks, months and hours. It is the fast way to find the days between two dates for a project timeline, a countdown, a notice period, a pregnancy due date, or simply how long ago something happened.

Pick a start date and an end date and the answer updates instantly — no submit button and no page reload. A 'Include the end day' toggle handles inclusive counting (the way you would count hotel nights or rental days), and an optional time-of-day mode adds hours and minutes for precise durations. Working out someone's age? Our Age Calculator adds a next-birthday countdown on top of the same math.

Calendar days, weekdays and business days

Not every count should include weekends. The business days calculator tab counts only Monday to Friday between your two dates and lets you subtract public holidays — choose a built-in United States, United Kingdom or Canada holiday preset, or paste your own dates. It handles deadlines, SLAs, shipping windows and payroll where working days between dates is what actually matters.

Each result separates calendar days, weekend days and the public holidays it excluded, and lists the holidays that fell inside your range so you can check the math at a glance. Timeanddate.com and calculator.net both offer business-day counting but require separate page loads and do not list which specific holidays were excluded — you just get a number and have to trust it.

Add or subtract days, weeks, months and years

Need a future or past date instead of a gap? The add or subtract tab takes a starting date and shifts it by any combination of years, months, weeks and days, returning the resulting date and its weekday. Use it to find a 90-day deadline, a 30-day return window, or a date six weeks ago. Switch on 'business days only' and it counts forward skipping weekends — useful for delivery estimates and contract turnaround times.

The math handles month-end edge cases properly. Adding one month to 31 January gives 28 February (or 29 in a leap year), not an invalid date — the same way real-world calendar math works, not the simplistic 30-days-per-month shortcut many tools use.

Days until or since a date

The quick 'days until / since' box compares any date to today: a future date shows how many days until it arrives and the day of the week it lands on, while a past date shows how many days since it passed. It is useful for counting down to a birthday, a wedding, an exam, a product launch, or a holiday — all without leaving the page.

Timeanddate.com has a similar countdown feature but requires a login to save countdowns and shows banner ads around the result. Calculator.net has a basic days-between page but puts business days on a separate URL. Here the answer is free and immediate on the same screen, and you can bookmark or share the URL with your date pre-filled.

How the calculation works — and leap years

The total-days figure is the exact number of calendar days between the two dates. The years-months-days breakdown borrows the true length of each calendar month, so it stays accurate across short and long months — which is why the breakdown and the raw day count can look different even though both are correct. Leap years are handled automatically: 29 February is counted whenever a leap year falls inside your range, so a span that crosses one is one day longer than the same span in an ordinary year.

The business-days calculation loops through every date in the range, checks whether it is a weekday, and checks whether it matches a holiday in your selected list. This approach is slower than a formula but handles irregular holiday schedules — including countries where a holiday falling on a weekend is observed on Monday — without requiring you to do any manual adjustment.

Free, private and works on every device

Everything — the date difference, the business-day count, the add/subtract result and every total — is computed locally in your browser. No dates are uploaded, nothing is stored on a server, and there is no sign-up. You can copy a plain-text summary or share a link that reopens the calculator with your exact dates and settings already filled in.

The calculator works on iPhone, Android, and any desktop browser. Unlike timeanddate.com — which serves a desktop layout on mobile and loads slowly — this tool is mobile-first and opens immediately. No ads inside the calculator, no paywalls, no limits on how many calculations you run.

Why month and year differences are inherently ambiguous

Counting the days between two dates is unambiguous — you subtract one Unix timestamp from the other and divide by 86,400. Counting months between two dates is not, because months have unequal lengths and the convention you use produces different answers. The two most common approaches are: calendar month counting, where you advance the month counter whenever the same day-of-month is reached again (January 31 to February 28 = zero full months plus 28 days, because the 31st of February never arrives), and the last-day-of-month convention, where hitting the last day of the destination month is treated as completing the month regardless of the day number (January 31 to February 28 = exactly one month, because February simply has no 31st).

Age calculation uses a strict form of calendar month counting: your n-th birthday is the anniversary of the exact day-of-month you were born, and you complete n years only when that day arrives. This matters most for people born on February 29 (a leap day). Different jurisdictions handle leap-day birthdays differently — most common-law countries treat February 28 as the legal birthday in non-leap years, while some civil-law countries use March 1. The practical consequence is that two people born 24 hours apart on February 28 and March 1 of a leap year may reach a given legal age on different days in ordinary years.

Excel's DATEDIF function implements yet a third interpretation, and its behaviour for the 'MD' (days ignoring months) and 'YD' (days ignoring years) units is officially documented by Microsoft as containing known bugs when the start day is larger than the end day. For any calculation where the exact month count matters — a warranty period, a subscription billing cycle, a statute of limitations — it is worth verifying which convention your tool uses rather than assuming the answer is self-evident.

Calendrical edge cases: Julian gaps, proleptic Gregorian, and DST

The Julian to Gregorian calendar transition means that historical date arithmetic before 1582 is not straightforward. The Gregorian reform dropped either 10 or 13 days depending on when a given country adopted it — Britain and its colonies removed 11 days in September 1752 (September 3 through 13, 1752 never existed in those territories), while Russia did not switch until 1918. If you calculate the days between a medieval date and today using only Gregorian arithmetic, you will be off by up to 13 days. Most programming libraries, ISO 8601, and this calculator use the proleptic Gregorian calendar — extending Gregorian leap-year rules backward before 1582 — which gives internally consistent arithmetic but does not correspond to what a person in 1300 CE would have written in a diary.

Daylight saving time (DST) introduces a different category of edge case for hour-level calculations. When clocks spring forward, a day is only 23 hours long; when they fall back, it is 25 hours. If you measure the difference between two dates in local time across a DST boundary, the result in hours will not be a multiple of 24 even though the calendar-day count appears correct. For example, 'one week' from a date two days before the spring-forward transition is 167 hours, not 168. The reliable fix is to perform all date arithmetic in UTC and convert to local time only for display — the approach used by every serious date library and by this calculator's internal engine.

A subtler gotcha is time zone offset transitions that are not DST: countries that permanently shifted their UTC offset (for example, Samoa skipping December 30, 2011 entirely when it moved from UTC−11 to UTC+13) can remove or duplicate a calendar day. These cases are rare, but they make clear why the phrase 'days between two dates' depends on which calendar, which time zone, and which convention you are using — the question is only unambiguous once all three are specified.

Date arithmetic in programming: bugs, libraries, and the Temporal API

Date arithmetic is one of the most reliably bug-prone areas of software development. JavaScript's built-in Date object is a notable example of the hazards. Months are zero-indexed — January is 0 and December is 11 — so new Date(2024, 1, 15) is February 15, not January 15, a trap that catches experienced developers. Strings are interpreted differently depending on format: new Date('2024-01-15') is parsed as UTC midnight, while new Date('01/15/2024') is parsed as local time midnight. Display it in a UTC−5 timezone and the ISO-format date appears to land on January 14 — a classic 'off by one day' bug that has corrupted more than a few birthday lookups and appointment systems.

The moment.js library was created specifically to paper over these issues, and for years it was the default answer for date arithmetic in JavaScript. The team eventually declared the project complete and recommended migrating away from it — the library's mutable API and its enormous bundle size (232 kB minified) became liabilities in modern web development. Its successors each make a different trade-off: date-fns is tree-shakeable and functional; Luxon is immutable and wraps the Intl API for time zone handling; Day.js is tiny (2 kB) with a moment-compatible API. The ECMAScript Temporal API, now at Stage 3 in the TC39 proposal process, will eventually land in browsers natively — it introduces separate types for calendar date, wall-clock time, and instant, which prevents the UTC/local confusion that plagues the current Date object.

The pattern repeats across languages. Python's datetime.date handles subtraction cleanly (two dates subtracted return a timedelta object) but cannot do month arithmetic without the dateutil library. Go's time.Time includes a AddDate method but silently normalises overflow — adding one month to January 31 gives March 2 (or 3 in a leap year), not February 28. The consistent lesson is that reliable date arithmetic — especially across months, time zones, and historical dates — requires deliberate use of a well-tested library rather than arithmetic on raw numbers.

Legal and contractual date calculations

Legal documents treat date differences with a precision that casual arithmetic often misses. A '30 days' notice period in a contract almost always means 30 calendar days, not 30 business days, counted from the day after the triggering event — so notice served on January 31 expires on March 1 (or March 2 in a non-leap year), not February 28. The distinction between 'before', 'on or before', 'after', and 'on or after' in statutory language moves the count by exactly one day in each direction, and courts have decided cases on that single day. Always confirm which convention applies before relying on a calculated date for a legal deadline.

Statutes of limitations are typically measured in years from the date of the event using the anniversary rule: a three-year limitation on an event that occurred on July 4, 2021 expires at the end of July 4, 2024 (or sometimes on July 5, 2024, depending on whether the jurisdiction counts the event date itself). Warranty periods expressed as '1 year from purchase' collide with the month-length problem: a product purchased on August 31 nominally expires on August 31 of the following year, but a product purchased on January 31 with a one-month warranty expires on February 28 or 29 — not a non-existent February 31.

Landlord-tenant notice periods illustrate how jurisdiction-specific the rules are. England's Section 21 notice requires 'at least 2 months' notice to end an assured shorthold tenancy, which the courts interpret using the calendar-month rule: 2 months from March 14 = May 14, not 61 or 62 days later. New Zealand requires 42 days (6 weeks). The United States varies by state and tenancy length — typically 30 or 60 days. The business-days tab on this calculator does not substitute for legal advice, but it can tell you exactly how many calendar days sit between two dates so you can verify a notice period against the rule that applies in your situation.