ClockSync is a native iOS and macOS app for configuring Xiaomi LYWSD02 (35911) and MJWSD05 clocks using Bluetooth.
TLDR;
Run the app and bring it close to your clock. If it’s a LYWSD02, it should show up automatically. If it’s a MJWSD05, you will need to also press the lower button on the right to put it into bluetooth discovery mode. From there the app should show a configuration screen which hopefully will be self-explanatory.
Detailed usage information can be found on the help page.
Credit Where Credit Is Due
ClockSync is all original code, but builds on public reverse-engineering work and open source client implementations for the Xiaomi LYWSD02 family.