Quiz system clearity

I am developing a quiz system for an LMS in production for a University, I have designed a rough sketch, I am not sure about a scenario where a student say, requests a quiz how should I cover the quiz start time and end time. One option is to store the time start when the request is made on server side and the once the user gets the display say, on document ready should I fire some API and get start time again? same goes for the end time. What if the students request and sees the quiz and because of say, some internet problem could not submit the solution and what if he/she does so on purpose.

And what do you think of PHP similar text function to compare short answer questions (incase a teacher adds a short question with an answer) is it feasible to check the solution submitted by student against the answer added by the teacher and keep a percentage for correctness say the teacher while adding question adds 80% and above as correct...