I can't speak for the app on Android devices, but I have been using it on my iPhone 6s, and I haven't noticed any issues with the timing. I have been using an analogue watch to pace myself, and each section seems to end exactly when expected.

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I personally prefer to reset it before each section.

You should have plenty of time, since between sections the proctor will say, "you will have the next 35 minutes to work in your test-book and answer sheet for section # only. During this time you may not read, work on, make marks or erasers in, or turn the page to, any other section".