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Designed this because I just wanted a simple count up / down timer that runs on almost anything with a browser - enjoy.
If the display shows 00 and you press Start, the timer counts up. If you set a time with the arrow buttons and press Start, it counts down. When the countdown reaches zero, it keeps going and shows the total elapsed time.
On some iPhones and Android phones, you may be able to install this as a web app using your browser's Add to Home Screen option. That can give you a cleaner, more fullscreen view with less browser UI visible.
Disclaimer: This timer is provided "as is" with no guarantees. Results may vary by device / browser, and backgrounding the page may pause timing.