I can't speak to the origins, but Kelloggs does use "the best way to start a day" as a slogan, not just to describe their own brands but about cereal in general

I can't speak to the origins, but Kelloggs does use "the best way to start a day" as a slogan, not just to describe their own brands but about cereal in general: https://www.kelloggcompany.com/content/dam/kellogg-company/files/CerealTheCompleteStory_FINAL.pdf https://www.kelloggcompany.com/content/dam/kellogg-company/f...