Will Smith wears a Hamilton Ventura

Agent J got recruited into the Men in Black in 1997 by Tommy Lee Jones's Agent K. He's been wearing a black suit, sunglasses, a neuralyzer, and a Hamilton Ventura ever since. Will Smith plays him through Men in Black 3 (2012), the entry where J has to time-travel to 1969 to save his partner.

The Ventura is the suit-and-tie of the franchise, as fixed a part of the uniform as the dark glasses. Tommy Lee Jones's K wears one in 1997, and in Men in Black 3 both Will Smith's J and Josh Brolin's younger K wear Venturas, in the present day and back in 1969. Hamilton has made the Ventura more or less continuously since 1957, so the costume can follow an agent into any decade and still be the right watch. On J's wrist the watch is the chronograph version, the Ventura Chrono.

What the Ventura does for J is mark him as part of the inheritance, the same watch the agents before him wore. It is one of two everyday objects the franchise turned into part of its look. The other is the Ray-Ban Predator sunglasses every agent hides behind, popular enough that Ray-Ban's sales jumped after the first film. The suit is anonymous and the alien weapons are invented, but the watch and the glasses are things you can actually go out and buy. That is part of the appeal.