Students are not changing the time at which Moodle will accept their response. Students are only changing the helpful number displayed on their monitor. It is like this:
Student is sitting in an exam hall for a 3 hour exam, with a proctor/invigilator at the front who will tell people when to start and stop. At 10:00, the student is told to start, and the check their wristwatch, and see that also shows 10:00, so the exam will finish at 13:00.
At 12:00, the student thinks "Help, I am running out of time", and so winds their wristwatch back to 11:00. Now they have an extra hour, right?
Well, no. Of course not. There is still only 1 hour before the invigilator says stop. All that happens in that the student has confused themself about how much time is left.