No, you do not need to specify a time slot to take the Proctored Exam. You can take the exam at any time during the Proctored Exam period for that term, but you must complete the exam by the end of the exam period. If you start your exam within 75 minutes of the end of the exam period, you will only have until the end of the exam period to complete your exam, instead of the full 75 minutes.