"But a careful person, knowing the content of all those letters, could have worked out what you intended to do?"

"Yes, but no one person knew the contents of all the letters. Miss Winthrop and I were the only two human beings who knew what was in them all. Neither Miss Winthrop nor I left the office all day. Yet before the day ended, I received phone calls from two managers telling me of the unbeatable offers from my competitor."

Yet before the day ended,这里的yet看作连词吗？那么before这里应该看作副词前置吗？