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Weekday, week-number and interval arithmetic on the Gregorian calendar, including range overlaps, recurring weekday counts and…
Arithmetic on hours, minutes and seconds plus conversion across UTC offsets and daylight saving rules. Used for shift planning…
Conversions between the Gregorian calendar and the numbering systems used by software, astronomy and science: Unix epoch, Julian…
When the Sun crosses the horizon and how its geometry drives daylight, shadows and sundials. Used by photographers, architects…
Lunar phase and rise/set timing, the instants of equinoxes and solstices, and the tide timing that follows the Moon. Used by…
Kepler-law arithmetic for planets, moons, binary stars and satellites, plus transfer burns, tidal limits and relativistic radii…
The measurement chain of observational astronomy: light travel time, parallax distance, magnitudes and distance modulus, redshift…
Optical and imaging arithmetic for visual observers and imagers: magnification, focal ratio, exit pupil, resolving power, sensor…
Great-circle course geometry, coordinate projection conversion, map scale and classic celestial navigation. Used by mariners…
Most used in time, date, navigation & astronomy
- Day of the Week CalculatorZeller's congruence (1882); ISO 8601 weekday numberingEnter any date and this calculator returns the weekday, the ISO 8601 weekday number, the day of the year and the Julian Day…
- ISO Week Number CalculatorISO 8601-1:2019 week numberingThis calculator converts a date to its week number and back again. In ISO 8601 mode it returns the week number, the week-based…
- Age in Days, Hours & Minutes CalculatorFliegel–Van Flandern Julian Day NumberEnter a date of birth and this calculator returns the exact elapsed time to any reference date: total days, weeks, hours…
- Time Duration CalculatorISO 8601 time-of-day and duration conventionsEnter a start time and an end time and this calculator returns the elapsed span in hours, minutes and seconds, in decimal…
- Add & Subtract Time CalculatorSexagesimal (base-60) duration arithmeticAdd or subtract up to three durations written as hours, minutes and seconds, and get the answer in h:m:s, in decimal hours, in…
- Time Zone Converter CalculatorIANA Time Zone Database (tzdb) offset conventionsGive this calculator a date and time, the UTC offset of the zone it is written in and the UTC offset of the zone you want it…
- Daylight Saving Time Change Date CalculatorUS Energy Policy Act 2005; EU Directive 2000/84/ECDaylight saving transitions are defined by a rule, not by a published list, so you can compute them for any year. This…
- Unix Timestamp Converter CalculatorPOSIX.1-2017 seconds since the EpochPaste an epoch value and get the UTC date, your local date, the ISO 8601 string and the day of the week; or switch direction…
- Leap Year CalculatorGregorian calendar reform of 1582 (papal bull Inter gravissimas)Type a year and this calculator tells you whether it is a leap year, how many days it holds, and which years either side of it…
- Sunrise & Sunset Time CalculatorNOAA solar position algorithm (after Meeus, Astronomical Algorithms)Enter a latitude, longitude, date and UTC offset and this calculator returns sunrise, sunset, solar noon, day length and the…
- Daylight Hours & Day Length CalculatorNOAA solar position algorithm (after Meeus, Astronomical Algorithms)This calculator gives the hours of daylight at any latitude on any date, together with the change from the day before, the…
- Twilight Times CalculatorUSNO twilight definitions (−6°, −12°, −18° solar altitude)Twilight is defined by how far the Sun sits below the horizon, not by how the sky looks, and the three standard thresholds are…