If you have set alarm for 10 hours and 60 minutes, you can be 100% sure that alarm will ring at 10 hours and 60 minutes. You can trust us completely.
The alarm will still ring at 10 hours and 60 minutes, even if you refresh the browser, since we store the information in browser's local cache.
It will be gone. The alarm will not ring at 10 hours and 60 minutes, if you close the tab.
It will be gone. The alarm will not ring at 10 hours and 60 minutes, if you close the browser.