Rainn Wilson wears a Casio Databank

Dwight Schrute runs a beet farm, sells paper, and moonlights as a volunteer sheriff's deputy, and the watch for a character like that is a Casio Databank, reference DBC30-1: a black resin case, a keypad ringing the screen for storing phone numbers, and the "Telememo 30" text on the dial itself referring to how many entries it holds. Casio discontinued the model years ago; the closest current descendant is the DBC-32.

It works as a companion piece to Michael Scott's watch, also documented on this site: where Michael's is merely cheap, Dwight's is actively utilitarian, a tool before it's a timepiece, in keeping with a character who alphabetizes his beet harvest and keeps a weapon in his desk drawer. The clearest look at it comes in season two's "The Fire," when Dwight grabs Ryan by the collar during the office fire drill and the calculator watch is right there on the wrist doing the grabbing.