Other features include an hourly time beep, a single daily alarm lasting 20 seconds and an annual calendar.

[7] The watch uses a faint (though this can be improved with the use of an aftermarket modification[8]), green LED backlight located at the left of the display for illumination (in earlier units it was an amber microlight).

[7] 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 top left button is used to cycle through seconds, hours, minutes, month, date, day and normal mode.

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.

"[15] United States military intelligence officials have identified the F-91W as a watch that terrorists use in constructing time bombs.

[20] 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.

These counterfeits generally have a lower plastic build quality, narrower LCD viewing angles, louder and higher-pitched beeps, and significantly less accurate timekeeping than genuine models.

This leaves the only method of distinguishing them as assessing the overall build quality, timekeeping accuracy, display viewing angle and the printing on the screen glass.