A calculator watch was a digital watch, with a calculator built in, and buttons for keying calculations.
It represented several innovations over those available in 1955:
- all watches were analog in 1955 (as seen in the difference in the clock on Mr. Foster's Travel Agency between 1955 and 1985),
- it was electrical and had no moving parts (and therefore would not have made a ticking noise),
- it featured an electronic alarm, instead of a small internal vibrating clapper to make a ringing noise.
- the handheld calculator of the day was an analog mechanical device called a slide rule (which, though it could be fitted onto a wristwatch (in its circular form), would certainly not have buttons),
In spite of his adventures in time travel, Marty McFly's own wristwatch apparently was not able to keep the correct time. Whenever Doc Brown reminded Marty of the correct time, Marty would look at his watch, shake it and hold it to his ear as if he were listening for a ticking sound, even though electrical watches are silent.
When Marty tried to contact Emmett Brown using the payphone at Lou's Cafe, he had to abort the attempt when his watch alarm sounded with an electronic beeping noise. Lou Caruthers eyed him suspiciously, wondering what was making the unusual sound, and Marty wisely lowered his hand.
Behind the scenes
In one of the drafts of Back To The Future, the schoolteacher punishes Marty for talking in his class, by setting him a difficult calculation to solve by the end of the class. Marty surreptitiously slips out his pocket calcalutor and solves the problem in a matter of seconds. In the first draft, Professor (sic) Brown checks the distance required for reaching the atomic blast (what later was changed into the Clock Tower scene) using a slide rule.