// Rubber Duck Thursdays - It's time to build!

Chris showcases Timestamp (gh.io/timestamp), an open-source countdown timer app that evolved from the 2025 holiday streams into a fully extensible project with multiple themes, comprehensive testing, and GitHub automation.

Timestamp — Open Source Countdown Timer

The app is built with vanilla TypeScript and Vite, featuring:

• Multiple themes — contribution graph (GitHub-style squares that fill as midnight approaches), fireworks celebration (powered by fireworks.js), and a scaffoldable ring theme
• World map wall clock — an interactive SVG map using Natural Earth public domain data with day/night visualization
• Time zone support — countdown to midnight in any city worldwide with a drop-down and map selector
• Light/dark mode — system-aware theme switching with per-theme styling
• Accessibility — built-in reduced motion support, keyboard navigation, and WCAG considerations

Copilot Custom Instructions vs Agents vs Prompt Files

Chris shares key lessons learned from structuring his Copilot configuration:

• Instructions files = the onboarding playbook (coding standards that apply to all agents, like documentation conventions and GitHub Actions best practices)
• Custom agents = specialized team members (code quality specialist, UI performance specialist, testing specialist) with defined workflows, role boundaries, and stopping rules
• Prompt files = task entry points that compose instructions + agent behavior
• Separation of concerns prevents duplicated or contradictory guidance across files

Manager-Specialist Agent Pattern

Chris demonstrates a manager agent that orchestrates specialist sub-agents for spec writing, planning, implementation, and code quality review — each with structured JSON responses and defined phase workflows using the MoSCoW method (MUST/SHOULD/COULD) for requirement prioritization.

Testing at Scale

• 2500+ unit tests with Vitest covering time zone utilities, app state, theme rendering, and countdown logic
• 300+ end-to-end Playwright tests for cross-browser validation
• Automated preview generation using Playwright to screenshot themes in light/dark mode at multiple sizes

GitHub Actions Automation

• Automated issue form updates when new themes are added or removed
• Auto-labeling and assignment for bug reports and feature requests
• Theme scaffolding CLI (npm run theme create <name>) that generates config, renderers, tests, and registry entries
• Dependabot for weekly dependency updates, secret scanning with push protection, and CodeQL code scanning enabled

GitHub Features in Use

GitHub Actions, GitHub Codespaces (for contributor environments), GitHub Advanced Security (Dependabot, secret scanning, push protection, CodeQL), Octicons for UI icons, and structured issue templates with dynamic theme lists.