https://radicale.org/v3.html
&& sync
If you want to be more targeted:
blockdev --flushbufs /dev/sda
but it's a pre-bought circular screen, raspberry pi and a browser... i could use it to ask google the time, the clock is just a formality.
thinking about it a little harder, i guess the focus is not really the project and more so the watch faces.
https://radicale.org/v3.html