Zestimer is a HIIT interval timer application designed for iPhone and Apple Watch, built to guide users through timed workout routines without requiring manual intervention during exercise sessions. The app functions as a comprehensive timer that provides voice cues, beeps, and haptic feedback to announce each exercise and rest period. It includes a library of 224 illustrated exercise moves and 39 preloaded workout routines, with support for up to 10 custom workouts in its freemium tier. The application integrates with Apple Health and iCloud for data synchronization, offers music controls during workouts, and provides Home Screen widgets including Quick Start and heatmap displays. A distinctive feature of Zestimer is its ability to convert YouTube workout videos into guided, timed routines. Users can search for videos directly within the app, and the application analyzes the video content to identify exercises and time intervals, generating a follow-along workout that runs automatically. The app also includes an AI Coach feature that provides personalized coaching for each workout and can generate new workout routines from scratch using artificial intelligence. Additional capabilities include casting workouts to a TV or AirPlay display and unlimited custom workout creation for users who upgrade. The application was developed in response to the creator's experience with paywalled fitness apps that interrupted workouts. Zestimer operates on a freemium model where core functionality remains accessible without payment, and upgrade prompts appear only when users seek additional features. The app supports iOS 26 and is built with a Liquid Glass design aesthetic. Typical use cases include HIIT training, Tabata protocols, stretching routines, and circuit training, with the app managing the timing of each exercise interval and rest period from start to finish without requiring user interaction.