Business Hours Between Datetimes

The 40-hour workweek that anchors the standard 9-to-5 business day was codified in the United States by the Fair Labor Standards Act (FLSA) of 1938, signed by President Franklin D. Roosevelt on 25 June 1938. The Act established a 44-hour standard week (reduced to 40 by 1940) above which non-exempt workers must be paid time-and-a-half overtime. The 40-hour figure was the result of decades of labour movement organising, drawing on Henry Ford's pioneering 5-day, 40-hour schedule introduced at Ford Motor Company in 1926.

The 8-hour workday split into 9-to-5 with a one-hour unpaid lunch break originates in the late 19th-century ‘Eight hours for work, eight hours for rest, eight hours for what we will’ campaign of the International Workingmen's Association, popularised at the 1886 Haymarket protests in Chicago. The slogan became law in the US for federal contractors in 1916 (Adamson Act for railroads) and for all non-exempt workers under the FLSA in 1938.

Most countries now legislate business-hour limits. The European Union Working Time Directive (2003/88/EC) caps the average workweek at 48 hours including overtime, requires 11 hours of daily rest, and mandates a 20-minute break after 6 hours of work. France famously legislated a 35-hour workweek in 2000. The UK opted out of the 48-hour cap but otherwise follows the directive. Australia's Fair Work Act 2009 sets a 38-hour standard week.

Federal holidays in the US that fall on a weekday remove a business day from the calendar. There are 11 such holidays in 2026 by federal observance: New Year's Day (Jan 1, Thu), MLK Day (Jan 19, Mon), Presidents Day (Feb 16, Mon), Memorial Day (May 25, Mon), Juneteenth (Jun 19, Fri), Independence Day (Jul 3 observed for Jul 4 Sat), Labor Day (Sep 7, Mon), Columbus Day (Oct 12, Mon), Veterans Day (Nov 11, Wed), Thanksgiving (Nov 26, Thu), Christmas Day (Dec 25, Fri). This calculator lets you add custom holidays to the working-hours math.

The Federal Reserve banking system observes 11 holidays per year, mostly identical to the federal civilian holidays but with slight variations (Good Friday is observed by markets but not by Federal Reserve banks; Columbus Day is observed by the Fed but not by NYSE). For shipping, logistics, and clearing calculations, the Fed's holiday list at federalreserve.gov is authoritative.

The 24/7/365 always-on operating model that dominates digital businesses (Amazon, Netflix, Stripe, cloud infra) is a recent inversion of the historical norm. Even within those companies, headcount-intensive functions (HR, finance, support tier-2) still operate on a 9-to-5 business-hour schedule because of labour cost, sleep biology, and team co-presence dynamics. Business-hour math therefore remains the core unit of SLA contracts and customer-success commitments.

Custom shift schedules - 7-to-3 morning, 3-to-11 afternoon, 11-to-7 graveyard, four 10-hour days, three 12-hour weekends - are increasingly common in healthcare, manufacturing, energy, and call centres. The configurator on this calculator lets you set start hour, end hour, and which days of the week count. The math then accumulates only the overlap between your custom schedule and the [start, end] window.