The Casio F-91W is a digital watch manufactured by Japanese electronics company Casio. Introduced in June 1989 as a successor of the F-87W, it is popular for its low price, long battery life, and iconic design. As of 2011, annual production of the watch was 3 million units, which made it the most sold watch in the world.
Designed by Ryà «suke Moriai as his first design for Casio, the case of the F-91W measures . The case is primarily made of resin, with a stainless steel caseback and buttons, with the manufacturer's module number, 593, stamped on the caseback. The resin strap is at the fitting and 22 mm across the widest part of the lugs. The total weight is .
The F-91W has a chronograph, featuring precision of seconds with a count up to 59:59.99 (nearly one hour). The chronograph is also able to mark net and split times (i.e., laps). Other features include an hourly time beep, a single daily alarm lasting 20 seconds and an annual calendar. The watch does not make allowances for leap years as there is no provision to record the year. Due to this, the month of February is always counted as 28 days. The watch uses a faint green LED backlight located at the left of the display for illumination (in earlier units it was an amber microlight). According to the manufacturer estimates, the watch is reported to be accurate to ñ30 seconds per month.
The quartz movement, designated Module 593, is powered by a single CR2016 3-volt lithium button cell rated to a stated battery life of 7 years.
The front of the watch is marked "Water Resist". The black version (F91W-1) is "30 meter / 3 bar" (i.e., 100 feet / 44 psi), the ISO standard meaning of which is: "Suitable for everyday use. Splash/rain resistant. NOT suitable for showering, bathing, swimming, snorkeling, water related work and fishing".
The watch is controlled by three side-mounted push-buttons.
The time or date is adjusted by pressing the lower left button three times to bring the watch to time adjustment mode. The top left button is used to cycle through seconds, hours, minutes, month, date, day and normal mode. The right button is used to adjust the flashing value displayed. Unlike any other value, the seconds can only be zeroed. Should this happen before 30 seconds, the watch will zero in at the beginning of the current minute. After 30 seconds it will start the next minute as displayed. When the adjustments are finished, the bottom left button can be pressed once to return the watch to normal mode.
The watch display shows the day of the week, day of the month, hour, minute, seconds and the signs PM in the afternoon â or 24H (24-hour clock) â at all times, the alarm signal status (bar of vertical lines), and the hourly signal status (double beep on the hour, shown as a bell) are present when activated in the alarm mode.
In stopwatch mode, minutes, seconds and hundredths of a second are shown.
The US government became suspicious of Afghans who wore Casio watches due to their ability to be used as timers for improvised explosive devices, a tactic favored by al-Qaeda.
According to secret documents issued to interrogators at Guantanamo Bay, obtained and released by The Guardian, "the Casio F-91W digital watch was declared to be 'the sign of al-Qaeda' and a contributing factor to continued detention of prisoners by the analysts stationed at Guantanamo Bay. Briefing documents used to train staff in assessing the threat level of new detainees advise that possession of the F-91W and the A159W, available online for as little as ã4, suggests the wearer has been trained in bomb making by al-Qaeda in Afghanistan." United States military intelligence officials have identified the F-91W as a watch that terrorists use in constructing time bombs.
This association was highlighted in the Denbeaux study, and may have been used in some cases at Guantanamo Bay. An article published in The Washington Post in 1996 reported that Abdul Hakim Murad, Wali Khan Amin Shah, and Ramzi Ahmed Yousef had developed techniques to use commonly available Casio digital watches to detonate time bombs. Casio watches were mentioned almost 150 times in prisoner assessments from Guantanamo.
On 12 July 2006, the magazine Mother Jones provided excerpts from the transcripts of a selection of the Guantanamo detainees. The article informed readers:
The article quoted Guantanamo Bay detainee Abdullah Kamel Abdullah Kamel Al Kandari:
Counterfeits of this watch are very common, despite its low price. These counterfeits usually have lower-quality plastic, narrower LCD viewing angles, louder and higher-pitched beeps, and less accurate timekeeping than genuine models. On newer versions with the green LED light, holding the right button for more than 3 seconds in the main timekeeping mode will make the display show "CASIo" as a quick authenticity check. However, some counterfeit models have been designed to mimic this feature. Another way to check is by pressing all three buttons at once to enter test mode. In some fake modules, an extra dot appears on the display.