Date arithmetic looks trivial and is full of edge cases: months have different lengths, weeks start on different days in different countries, and 'two business days' means different things depending on where the person counting lives.
Subtract the earlier date from the later one, and decide whether both endpoints count. 'From Monday to Friday' is four days of difference and five days inclusive, which is the source of most off-by-one disputes.
Count the weekdays between them, then subtract public holidays that fall on weekdays. The holidays depend on the country, so the same two dates give different answers in London and New York.
A leap year has 366 days, with 29 February added. A year is a leap year if it divides by 4, except century years, which must divide by 400 — so 2000 was a leap year and 1900 was not.
Because century years are only leap years when they divide by 400, and 2100 does not. The rule keeps the calendar aligned with the solar year, which is about 365.2425 days rather than 365.25.
Conventions differ: most systems and many jurisdictions treat it as 28 February, some as 1 March. Calendar software usually skips the occurrence entirely, which is worth checking for anything that must happen annually.
ISO 8601 is the international standard for writing dates and times — 2026-03-14T15:00:00Z — ordered from largest unit to smallest. It sorts correctly as plain text and is unambiguous between regions, which is why software uses it.
Because it is 3 April in most of the world and 4 March in the United States. Any date written with slashes and no month name is a guess, which is why ISO format or a written month is safer in anything crossing borders.
A week number identifies a week within the year, from 1 to 52 or 53. Under ISO 8601, week 1 is the week containing the first Thursday of January, so early January can fall in the last week of the previous year.
Monday under ISO 8601 and across most of Europe; Sunday in the United States, Canada, Japan and much of the Middle East. Calendars should follow the reader's locale, and reports should say which they used.
52 weeks and one day, or two days in a leap year. That is why some years contain 53 ISO weeks and why a weekly event does not land on the same date each year.
Add the month, then clamp the day to the end of the target month — 31 January plus one month is 28 or 29 February. Systems that do not clamp roll over into March, which is a well-known source of quiet errors.
Subtract the birth year, then subtract one more if the birthday has not yet occurred this year. Comparing month and day rather than day-of-year avoids leap-year mistakes.
A duration is an amount of time — two hours, three days. An interval is a duration between two specific points. 'Three days' is a duration; 'from 4 to 7 March' is an interval, and only the second is affected by daylight saving.
Because daylight saving makes one day 23 hours and another 25. Adding 24 hours is therefore not the same as adding a day, and code that assumes it is breaks twice a year.
Start from the next business day after the trigger, then count forward, skipping weekends and holidays. Whether the start day counts is a convention worth stating explicitly in any contract.
Calendar days, including weekends and holidays, unless the document says business days. The distinction can be a fortnight's difference over a 30-day period.
A fiscal year is an organisation's accounting year and need not start in January — the UK tax year starts on 6 April, many companies use July or October. Reports comparing 'this year' across organisations are often comparing different twelve months.
A quarter is three months of a financial year. Q1 is January to March on a calendar year, but April to June for an organisation whose year starts in April, so a quarter needs its year stated.
Divide the minutes by 60 and add: 7 hours 45 minutes is 7.75. Timesheets and invoicing systems almost always expect the decimal form.
Convert everything to minutes, add, then convert back. Adding hours and minutes in separate columns is where the mistakes come from.
12am is midnight, at the start of the day, and 12pm is noon. The pair is confusing enough that 00:00 and 12:00 are worth using instead for anything that matters.
The start of the next, by convention. An event 'ending at midnight on Friday' is ambiguous enough that 23:59 Friday or 00:00 Saturday should be written instead.
The 24-hour clock runs from 00:00 to 23:59 with no am or pm, and is standard in most of the world and in all aviation and medicine. It removes the midnight and noon ambiguity entirely.
31 for January, March, May, July, August, October and December; 30 for April, June, September and November; 28 for February, or 29 in a leap year.
A Unix timestamp counts seconds since 1 January 1970 UTC, with no time zone of its own. It is unambiguous, which is why it is stored, and unreadable, which is why it is never displayed.
Systems storing Unix timestamps in a signed 32-bit integer overflow in January 2038. Modern systems use 64-bit timestamps, but the problem persists in old embedded software and some databases.
Add seven times the number of weeks in days, which always lands on the same weekday. Adding 'a month' does not have that property, which is why recurring plans are cleaner in weeks.
It is genuinely ambiguous — the coming Friday to some people, the one after to others. Any arrangement more than three days out is safer with a date attached.
Enormously: the UK has about 8, India has dozens varying by state, and the dates of Easter, Eid and Lunar New Year move each year. Assuming a working day in another country is a reliable way to schedule into a holiday.
Easter and everything anchored to it, Eid al-Fitr and Eid al-Adha, Lunar New Year, Diwali, Thanksgiving, and every holiday observed on 'the first Monday in' a month. Only a maintained holiday calendar gets these right.
Many countries observe it on the nearest weekday instead — the UK and US usually move it to Monday. Others simply lose it, which is why the number of days off varies year to year.
Start from weekdays, subtract the national holidays for the relevant country, then subtract any company days. A calculator that lets you supply your own list is the only way to get a company shutdown right.
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