Which of the following has not aided historians in dating events in the remote past

Read the excerpt from eighty years and more. until i was sixteen years old, i was a faithful student in the johnstown academy with a class of boys. though i was the only girl in the higher classes of mathematics and the languages, yet, in our plays, all the girls and boys mingled freely together. in running races, sliding downhill, and snowballing, we made no [division] of gender. true, the boys would carry the school books and pull the sleighs up hill for their favorite girls, but equality was the general basis of our school relations. i dare say the boys did not make their snowballs quite so hard when pelting the girls, nor wash their faces with the same [forcefulness] as they did each other's, but there was no public evidence of [favoritism]. which detail supports the central idea that stanton feels females and males were generally treated equally at johnstown academy? until i was sixteen years old, i was a faithful student . . i was the only girl in the higher classes of mathematics . . [t]here was no public evidence of [favoritism]. true, the boys would carry the school books . .