A number of watches are still sold with flight computer slide rules in the bezel, including my current favorite the Citizen Nighthawk, which is a chrono-less si

A number of watches are still sold with flight computer slide rules in the bezel, including my current favorite the Citizen Nighthawk, which is a chrono-less sibling to the Navihawk, which is something of a budget homage to the Breitling Navitimer. I find it quite handy in daily use, for example converting between Celsius and Fahrenheit, kilometers and miles, and currency conversions when traveling. I'll even use it sometimes when calculating a tip (the F<->C conversion of 1.8 conveniently aligns with a typical tip of 18%).