Very smooth. Is the code open source?

Seems to be simple enough (just plain JS not minimified, and CSS), that you can just look at the source view-source:https://www.clocksimulator.com/

Good ol' days spirit

Thank you. Code can be viewed at https://github.com/timoheimonen/clocksimulator

Thanks! I was keen to find out how you prevented screen sleep and was interested to find out about wakeLock.