Userform Count down timer do not run as it should when run for second time

Hi Forum Experts,

I am facing this strange problem while designing an exam test form. I allow users to complete test in maximum 10 minutes so I coded a count down timer of 10 minute using Application.Ontime method. It works fine. However it continue execution even if userform is unloaded (when user submits the test or cancel). On running the form for second time, count down starts from point when earlier form was unloaded but now with double the speed of ticking.
Can timer function (any function for that matter) be stopped from submit button click event?
Please help in fixing this. Thanks in advance for all the help.