Date arithmetic and time-zone questions in one place: how long until, how long since, and what time it is somewhere else right now.
«How many days left» looks like subtraction, but it is really a question about where you put the boundaries. Whether today counts as day one or day zero, whether the end date is included, whether you measure from midnight or from now — each choice shifts the answer by a day.
Add leap years, daylight saving, time zones and non-Gregorian calendars and it stops being subtraction entirely. The tools in this group handle each of those explicitly.
Several counting conventions coexist. International age increments on your birthday; year age is simply the current year minus the birth year; East Asian reckoning starts at one and increments at new year. Even where the law standardises on one, other domains — conscription, school years — keep using another.
So «how old are you» cannot be answered before deciding which convention applies. That is why the calculator shows several at once: different documents ask for different ones.
International age has its own edge cases: how a 29 February birthday behaves in common years, and which local date counts when the birth happened in another time zone.
Memorising that Seoul is thirteen hours ahead of New York will be wrong for several weeks a year. Regions that observe daylight saving move their clocks twice, and the transition dates differ by country. During the weeks when the United States has switched and Europe has not, the Europe–US gap is not what it usually is.
There are also places that never observe it, regions on 30- or 45-minute offsets, and countries that have changed their standard offset in recent years. «What time is it there» is a lookup, not something to memorise.
When scheduling across countries, convert into each participant's local time individually, and avoid the transition weeks if you can — those are when meetings go wrong most often.
The Gregorian calendar follows the solar year; lunisolar calendars follow the phases of the moon. The two cycles do not divide evenly, so lunisolar calendars insert a leap month, which is why a lunar birthday falls on a different Gregorian date each year — and why being born in a leap month is more complicated still.
The Hijri calendar is purely lunar and inserts no leap month, so Hijri dates move about eleven days earlier each Gregorian year. Ramadan takes roughly thirty-three years to travel through all the seasons.
Japanese era years are a separate axis again. The era changes on an imperial accession, so converting a year like «Reiwa 7» needs the start year of that era — and transition years are especially confusing because two eras share one calendar year.
Fixed-date holidays are simple. But holidays derived by calculation (Easter and everything anchored to it), holidays that follow a lunar calendar, and rule-based holidays such as «the fourth Thursday in November» all land on a different Gregorian date each year.
Substitute-holiday policy also differs. Some countries grant a weekday off when a holiday falls at the weekend; others simply lose it. That difference is easy to miss when checking whether an overseas counterpart is working.
Adding flight duration to departure time gives the wrong arrival, because origin and destination are in different zones. The arrival time printed on a ticket is local time at the destination; the actual duration has to be computed separately.
Crossing the date line can even produce an arrival date earlier than the departure date — routes from Asia to the US west coast are the classic case.
Airport departure planning runs the other way: work backwards from the scheduled departure through boarding cut-off, security and travel time to find when you have to leave home.
Time zone data and holiday rules are revised by governments. These tools compute from widely used standard datasets, but anything with legal weight — contract deadlines, immigration, filing dates — should be confirmed with the responsible authority.
Newly announced zone changes and one-off public holidays in particular can take time to appear. For dates that matter, check two sources.