I found this when I searched for documentation about configuring Apache.

Roughtime is a protocol that aims to achieve rough time synchronisation in a secure way that doesn't depend on any particular time server, and in such a way that, if a time server does misbehave, clients end up with cryptographic proof of it.

“Rough” time synchronisation means that, at this stage, we would be happy with time synchronisation to within 10 seconds of the correct time. If you have serious time synchronisation needs you‘ll want the machinery in NTP or even PTP (which needs hardware support to do right). There’s no reason why Roughtime shouldn‘t be (almost) as precise as NTP, but the use cases that we have in mind for now don’t need much precision. For example, about 25% of certificate errors shown by Chrome appear to be caused by bad local clocks and we don't need much precision to fix that.

Found via Cillian O'Driscoll's fork https://github.com/odrisci/roughtime
See also https://github.com/oreparaz/vroughtime for a simple client written in C.

Zephyr is a new generation, scalable, optimized, secure RTOS for multiple hardware architectures.
A number of boards are supported, including the Raspberry π Pico https://docs.zephyrproject.org/latest/boards/arm/rpi_pico/doc/index.html