Date & Time Tools

Date and time questions look simple — how many days until, what day of the week was that, how long between two times — and every one of them hides a convention. Does 'between' include the end date? Is a week five days or seven? Which year's week 44 do you mean, and does a shift that ends at 6 am stop at midnight? The tools in this category all answer the same way: the convention is stated on the page, next to the result, in plain words. Business days are counted in the interval (start, end] — the start day excluded, the end day included — ISO weeks follow the ISO-8601 rule printed on the Week Number page, and an overnight time span is read as ending the next morning. Nothing is silent and nothing is buried in a footnote, so when another calculator disagrees with one of these, you can see exactly which convention each of you used.

The twelve tools split into three jobs. The calendar tools handle dates: an exact age in years, months and days from a date of birth, the days between two dates in calendar and business terms, adding or subtracting days, weeks, months or years, the weekday and day-of-year of any date, a countdown to a chosen day, ISO week numbers converted in both directions, and the leap-year rule with its century exceptions worked step by step. The clock tools handle times of day: the duration between two clock times, a full weekly time card with unpaid breaks deducted and overtime split at 40 hours, and an hours adder that totals mixed-format lists — clock style, decimal hours, h-and-m notation — into the decimal form payroll needs. The live tools handle the world's clocks and the moment in front of you: a time-zone converter with real daylight-saving rules, and a stopwatch with laps beside a countdown timer.

Two engineering rules keep the answers honest. First, all calendar arithmetic is done on whole days converted to day numbers, never on timestamps, so a daylight-saving change — which steals an hour in spring and repeats one in autumn — can never drag a result onto the wrong date, and a 90-day span counted across a clock change is exactly 90 days. Second, clock and zone work uses the machinery built for it: minutes since midnight for durations, and your browser's own Intl time-zone database, the same IANA data that airlines and operating systems use, for offsets — applied on the date you ask about, not today's date, because New York's offset in November is not its offset in July. And the privacy rule that applies across Kwikfig applies here too: dates of birth, time cards and deadlines are calculated entirely in your browser and are never uploaded, stored or tracked.