How every number on this site is derived, and the primary sources we check it against.
Time is one of the few measurements where the answer is exact, not estimated. The hour, the day, and the calendar year are defined by international agreements that anyone can read. Everything on this site reduces to those definitions plus simple arithmetic. Where a number depends on a convention (e.g. the U.S. work year), we name the convention and link to its source.
The SI second is defined as the duration of 9,192,631,770 periods of the radiation corresponding to the transition between two hyperfine levels of the cesium-133 atom. One hour = 3,600 seconds. One day = 86,400 SI seconds. Civil days occasionally include a leap second to keep UTC aligned with Earth's rotation; we use the 86,400-second civil day for all calculations.
Source: BIPM, The International System of Units (SI) Brochure
We use the Gregorian calendar (introduced 1582), the international civil standard. A year is a leap year if it is divisible by 4, except for century years which must also be divisible by 400. So 2000 was a leap year; 1900 was not. Regular year = 365 days = 8,760 hours. Leap year = 366 days = 8,784 hours.
The 2,080-hour standard U.S. work year comes from 52 weeks × 40 hours, derived from the Fair Labor Standards Act of 1938 (29 U.S.C. § 207), which sets 40 hours as the threshold above which non-exempt employees earn overtime. Actual hours worked vary by industry, country, and contract; for international comparisons we reference OECD Average Annual Hours Worked.
Time-of-day calculations rely on the IANA Time Zone Database (also called tzdata or zoneinfo), the global authority on time-zone rules and historical DST transitions. The site uses the browser's built-in Intl API, which sources its rules from tzdata. Because DST rules change periodically (governments add or remove DST), recent transitions depend on your device having an up-to-date tzdata.
Month-level calculations use the actual length of the named month: 28 or 29 (February), 30 (April, June, September, November), or 31 (January, March, May, July, August, October, December). When a calculation needs an "average month," we use 365.25 ÷ 12 = 30.4375 days = 730.5 hours, which accounts for the leap-year cycle.
Calculator pages display a "Last reviewed" date. A review means each numeric claim and source link on the page has been re-checked against its primary source on that date. Pages are reviewed at least annually, and immediately whenever a relevant standard changes (e.g. a new FLSA threshold, a leap-year year transition, a U.S. state changing DST observance).
When a source is updated and the underlying number changes, we update the page and bump the "Last reviewed" date. We do not silently change numbers without updating the date.
If you find an error — a wrong number, a broken source link, an outdated claim — please report it to hello@howmanyhoursin.com with the page URL and what you believe is incorrect. Verified errors are fixed within 7 days and the "Last reviewed" date on the affected page is updated.
See also our About page for the editorial standards behind this site.