Make a list of the tasks being asked of you. Add an estimate of the number of hours you think those tasks will take( try to be as honest as possible, don't try

Make a list of the tasks being asked of you. Add an estimate of the number of hours you think those tasks will take( try to be as honest as possible, don't try to inflate them or keep them low to try to hit a date). Share that with your manager or whomever is imposing the unrealistic deadlines. Ask them to help you prioritize these tasks. Show them where you think you will be by the deadline they set, and how much longer you think it will take.