Methodology

Exact, expected and approximate results

Results carry a label so you know how much they depend on outside factors.

“Exact” describes the arithmetic, not a guarantee about how a particular employer, court or authority will count a deadline. For anything with consequences, check the rule that applies to you.

Calendar basics

• All dates are calendar dates with no time of day, so time zones and daylight-saving changes never affect a date calculation.
• The Gregorian calendar is used for every date, including dates before it was introduced in 1582 (the proleptic Gregorian calendar). Old documents dated in the Julian calendar will not match.
• Leap years: a year divisible by 4 is a leap year, except century years, which are leap years only when divisible by 400. 2000 was a leap year; 1900 was not.
• The calculation engine supports years 1 to 9999, the full range of the days-between and date calculators; the date pickers of some other tools offer a narrower range suited to their purpose. Impossible dates, such as 31 April or 29 February 2027, are rejected rather than silently corrected.

Age

Age is counted by anniversaries: first whole years, then whole months, then the remaining days. Each monthly anniversary is computed from the birth date itself, and an anniversary that does not exist in a shorter month falls on that month’s last day.

The age calculator also shows total days, total weeks plus remaining days, total months, and hours as days × 24 (approximate, because the time of birth is unknown). For people born on 29 February, the birthday in a common year is 28 February by default; you can switch to 1 March, as conventions differ by country.

In Hijri mode, the same anniversary method runs on Umm al-Qura months. A birthday on the 30th of a month falls on the 29th in years when that month has 29 days. Hijri age is available for dates between 1300 and 1600 AH.

Days between dates

By default the count runs from the start date to the end date without counting the start day, which is the number of midnights crossed. The “Include the end date” option counts both endpoints.

Weekdays are Monday to Friday; public holidays are not removed here (the business days calculator does that). Decimal months and years divide the day count by the average Gregorian month (30.436875 days) and year (365.2425 days), so they are approximations. If the end date is before the start date, the absolute difference is shown with a notice.

Adding and subtracting dates

Years and months are applied first, then weeks and days. If the resulting day does not exist, it moves to the last day of the month, and the result says so.

• 31 January 2026 + 1 month = 28 February 2026
• 31 January 2024 + 1 month = 29 February 2024 (leap year)
• 31 January 2026 + 1 month + 1 day = 28 February + 1 day = 1 March 2026
• 31 March 2026 − 1 month = 28 February 2026

Business days

• Add mode: the start date is never counted. Monday + 4 business days = Friday.
• Count mode: the start date is excluded by default, so Monday to Friday of the same week = 4 business days, or 5 with “count the start date”.
• Weekend: any combination of weekdays can be marked as weekend; the default is Saturday and Sunday.
• Holidays on a weekend are counted once, as a weekend day, never twice.
• Extra days off (company closures, regional holidays) can be added by hand.
• Calculations are limited to 10,000 business days.

State, regional and employer holidays differ, so check your contract or employer for the days that apply to you. Holidays that depend on moon sighting are not computed as rules.

ISO week numbers

Week numbers follow ISO 8601: weeks start on Monday, and week 1 is the week that contains the year’s first Thursday (equivalently, the week containing 4 January). A year has 52 or 53 ISO weeks, so the first or last days of a calendar year can belong to a week of the neighbouring year.

Hijri calendar (Umm al-Qura)

• Hijri dates use the Umm al-Qura calendar, the official civil calendar of Saudi Arabia, as built into modern browsers through the Unicode CLDR/ICU calendar data. Gregorian to Hijri is a direct lookup; Hijri to Gregorian finds the first day of the Hijri month in the same data, so both directions always agree.
• Example: 6 October 2026 is 25 Rabi al-Akhir 1448 AH on the Umm al-Qura calendar.
• Supported range: 1300 to 1600 AH (1882 to 2174 CE). Months have 29 or 30 days, and impossible dates such as the 30th of a 29-day month are rejected.
• If a browser does not support the Umm al-Qura calendar, the tools show a message instead of guessing.
• Why Islamic dates are “expected”: Umm al-Qura is computed in advance from astronomical criteria. Many countries start each month after an official moon-sighting announcement, so the official date there can differ by a day (countries such as Morocco can be one day behind). A per-country difference is shown only once it has been officially confirmed and recorded with its source.

Occasion countdowns

Fixed occasions count down to local midnight at the start of the day, in your own time zone.

Easter uses the anonymous Gregorian algorithm (Meeus/Jones/Butcher), which gives Western Easter for every Gregorian year: 5 April in 2026 and 28 March in 2027. Orthodox Easter is not computed.

Time calculator

Durations are counted in seconds and hours are not wrapped at 24, so 20:00 + 5:00 = 25:00 and 1:45 + 2:30 = 4:15. For the time between two clock times, an end time earlier than the start time is assumed to be on the next day: 22:00 → 06:00 = 8 hours. Equal start and end times give zero, not 24 hours.

Clock-time spans are measured on a 24-hour dial with no date attached. On the night a daylight-saving change happens, the real elapsed time of an overnight span is one hour longer or shorter than the result.

Work hours and time cards

• Time worked = end time − start time − unpaid break. Overnight shifts are supported, and a break cannot be as long as the shift.
• Decimal hours are rounded to two decimals: 7 h 45 min = 7.75, 20 min = 0.33.
• On the time card, daily overtime is counted first, then weekly overtime applies only to the remaining regular hours, so no hour is counted twice. Gross pay = regular hours × rate + overtime hours × rate × multiplier, rounded to the cent.

Example: five days of 08:00–17:30 with a 30-minute break, daily threshold 8 hours, weekly threshold 40 hours, rate 20, multiplier 1.5.

Thresholds and multipliers are always your inputs, never assumptions. US federal law (FLSA) generally requires at least 1.5 times the regular rate for non-exempt employees for hours over 40 in a workweek, and some states add daily rules; your contract, employer or local law decides what applies.

Countdowns and timers

• A custom countdown targets the date and time you choose in your device’s time zone and shows days, hours, minutes and seconds, rounded down.
• Timers are anchored to an end timestamp: when you start or resume, the end time is fixed, and the display is recalculated from it. A timer therefore keeps exact time even when a background tab is slowed down by the browser.

“Today” pages and time zones

Pages that depend on today’s date, such as the Hijri date today, the current week number and the occasion countdowns, are first rendered for a reference time zone, then updated in your browser with your device’s clock and time zone.

Testing and corrections

Automated unit tests compare the calculation engine with known reference values: leap years, month-end clamping, anniversary-based differences, inclusive and exclusive counts, business days and the French and US holiday lists (including observed dates), Easter dates, ISO weeks and 53-week years, Umm al-Qura conversions in both directions, 29 February birthdays, time spans across midnight and overtime without double counting.

The calendars, standards and laws behind these rules are listed on the sources page, and every change to a calculation rule or data file is recorded in the changelog. If you find a result you believe is wrong, use Report an error under the tool or the contact page.