Date and time calculators

One calendar, one date format

The Gregorian calendar, introduced by Pope Gregory XIII in 1582, is the civil calendar almost everywhere today. It adjusted the older Julian calendar, which added a leap day every fourth year without exception and so drifted against the seasons by about three days every four centuries. The Gregorian fix drops three leap days in each 400 years: century years are leap years only when divisible by 400. That is why 2000 had a February 29, while 1900 had none and 2100 will have none either. Countries adopted the reform at different times; Britain and its colonies did so in 1752, skipping 11 days to catch up.

All the date tools on this site use that calendar for every date, even dates earlier than the switch in a given country. For modern dates this makes no difference. For old records it can.

The same day can be written in several orders, and the short numeric forms are easy to misread across borders:

ISO 8601 is the international standard for writing dates and times. Its year-month-day order cannot be confused with the other two, and dates written that way sort correctly as plain text. The holiday box in the business days calculator expects dates in that form for the same reason.

Clock time and time zones

Clock time around the world is set relative to Coordinated Universal Time (UTC), a time scale run from atomic clocks. Local time is UTC plus or minus an offset, usually a whole number of hours but sometimes 30 or 45 minutes. Offsets change when a region starts or ends daylight saving time, and they change when a government rewrites its rules. The IANA Time Zone Database tracks that history for every region, and browsers and operating systems ship a copy of it, which is how the converter knows the offset for New York in March or Sydney in December without asking you.

Durations are simpler: an hour is 60 minutes wherever you are. The one catch is the clock change itself, when a night is an hour shorter or longer than the clock face suggests.