One address for everything.
Email contact@radialtimers.com. It goes straight to the maker, and every message is read.
Bugs. Say what you did, what you expected, and what happened instead. The browser and device help, and a screenshot helps more. If a timer chimed late or not at all, mention whether the tab was in the background, since that is the usual cause and it points to the fix.
Ideas. A feature you wish the timers had, a setting that gets in your way, a page that could say something better. The shortest way is the Suggest box on the front page, reached from the Suggest link in the footer of any page; it takes a note and an optional email for a reply. Email works just as well.
Timer requests. If the length you need is missing from the list of every timer, or a use you have in mind deserves its own page, say so. Landing pages get added when enough people ask for the same one.
Press and teachers. Questions about the site, its history as a YouTube channel, or using it in a classroom are welcome, and so are requests to quote from a page or reuse a screenshot.
Accounts and privacy. You can delete your account yourself from the account dialog, and the privacy page explains what the site keeps. Anything it does not answer, ask.
Built and run by Filbert Ephraim Wu (PHCuber on YouTube). The channel is where the rings started, and the same timers still run there as videos; comments on those videos reach the same person, but email is the reliable route.
Replies come from a person, not a queue, so they arrive when the maker sits down to answer mail rather than on a schedule. Bug reports that block a timer from running get read first. If you wrote in through the Suggest box without an email, there is no way to reply, so the change itself is the answer: watch News, where the things that ship get announced.
About tells the story of the site. Guides are the longer reads on using timers well. Terms and Privacy are the rules of the road.