Cell Formula to subtract 1 day from date

Column A= Date Started, Column B= Time Started, Column C=Date Ended, and Column D=Time Ended. I would like to know how to have 1 day subtracted from the Date Started Column (Column A) if the Time Started (Column B) is between 7:57pm and 11:59pm. I have formulated the Date Ended Column (Column C) to add 1 day to the date if the Time Ended Column entry is after 12:00am. This is a small sample of a larger program. Thank you.