With over 3 Million produced per year the Casio F-series is line of quartz digital watches, manufactured by the Japanese company Casio Computer Co., Ltd. The original F-91W was introduced in 1991 and is still in production and demand. It is popular for its simplicity, reliability and unpretentious, clean design.
Considered by many to be the World’s Most Versatile And Dangerous Watch…
The F91W has a 1⁄100 second stopwatch with a count up to 59:59.99 (almost one hour). Marking net and split time (lap) is also featured. There are the options of an hourly time beep and a single daily alarm lasting 20 seconds. It has an automatic calendar, although auto-adjustment for leap years is not supported, as the watch does not record the year. February is always counted as 28 days.  The watch uses a green LED backlight located to the left of the display to illuminate it. The light is not very even or bright but is plenty for reading the display easily. The watch is reported to be as accurate as ±30 seconds per month (6 minutes per year) by the manufacturer. As with every quartz movement, certain series of production units are more accurate.
The watch is powered by a single CR2016 3-volt lithium button cell, which will last at least seven years, assuming 20 seconds of alarm and one second of light usage per day. However, these are seldom used in practice, so it is not uncommon that the battery would last even longer. The watch case measures 37.5 × 33.5 × 8.5 mm and weighs 21 g. The manufacturer’s module number for this model (stamped on the stainless steel rear of the watch case) is 593.