When Martin shows Tim the alien gum, it's sealed under glass. But when Martin takes out the Venox 7 gum, the cover is gone with no time for it to have been taken off.
Ray Walston, who plays Nineenert, played the original Uncle Martin in the TV show "My Favorite Martian." In the movie he says his antennae are always giving him problems and everything is in black and white and that his gum had lost its flavor in '66. The original TV show was in black and white and its final season was in 1966.