Marty McFly's digital watch never seemed to have the correct time. Even though it had no moving parts and didn't "tick", Marty had a habit of looking at his watch, holding it to his ear and shaking it whenever Emmett Brown told him the correct time.
When portraying the alien being "Darth Vader" to convince his future father George McFly to ask Lorraine Baines to the Enchantment Under the Sea dance, Marty treated his digital watch like a communicator, listening to the watch's alarm and telling George "Silence! I am receiving a transmission from the Battlestar Galactica!"
The LCD digital watch would not be invented until 1972, by the Swiss manufacturer Longines, which introduced the watch in early 1973.