Back in his Army days, Barry Neistadt of Baltimore learned an acronym for the time before sunrise, and after sunset, when there was just enough light to see. But he's forgotten it, and figured I'd remember. Hah! I looked it up. He's probably thinking of BMNT (begin morning nautical twilight) and EENT (end evening nautical twilight). Both are times to "stand to" with heightened security. The practice dates from the French and Indian War, when both sides exploited the low light to launch attacks.