how to Calculate Time Taken of the Exam in Asp.net exam page

Question

how can i take the overall time taken of the exam in asp.net .

the time count is start when the student click start test button 

when i reload the page the time value should not be changing 

finally when the Student click the Finish Test button i need to get the total Time Taken value and need to save the database . 

 

i have try a lot but i did not got exact output .

so how can i do this , anyone know please help me to do that