Why does lincoln begin his eulogy to the soldiers buried at gettysburg with a reference to four score and seven years ago what significant event happened in america 87 years before 1863?

Lincoln’s main objective in his Gettysburg Address was to unite the nation again and regain the purpose of being a strong country. Because of that, he began his eulogy with the phrase “four score and seven years ago” which referenced to the signing of the Declaration of Independence, the major American symbol of Union and strong country.