ATC Calc Plus displays the time in large digits that are easy to read, even in bright sunlight when the rest of the display is hard to read. The buttons for entering the time and adding transit times, bogey times, etc. are large to make it easier to enter them on rough transits. Once the arrival time has been calculated, it takes two presses of the Clear button in order to reduce the chance of accidentally deleting the calculated arrival time because of a finger slip on a rough transit.
All of these features have been tested on stage in real rally conditions.