A timed delivered assessment's begin and end times need to be enforced across sessions. For example:
1) Student begins a 60min. timed assessment at 8:00 am.
2) Student partially completes assessment and closes the browser to go study up for the assessment.
3) Student reopens the browser and resumes timed assessment at 8:30 am.
4) System now only gives student 30min to complete the timed assessment. The timer bar should reflect this accurately as well.
PS - This is the same way Navigo was implemented.