A 17-year-old from Union County was charged Thursday, nearly two months after he posted a bomb threat on Twitter referencing Cranford High School, authorities said. A staff member saw the post and notified the school’s principal at about 5:40 p.m. on Feb. 8, Cranford police said. The principal called police, who enlisted the help of the Union County Sheriff’s Office K-9 unit and swept the property after evacuating the few people still inside the building at that time of day. The threat was deemed not credible. Investigators found the teen at his home in Cranford after police worked with intern…