Control systems, or any form of embedded safety critical system, use monotonic clocks where calendar time is a complete non-issue. Ordering of events, on what

Control systems, or any form of embedded safety critical system, use monotonic clocks where calendar time is a complete non-issue. Ordering of events, on what scale are we talking here? If it’s just within a transactional database there are multitude of ways to do it. Even distributed dbs have such features without relying on perfect time. If you are looking at a spanner style db you need a lot more guarantees than “I just used the time my cloud provider assigned to my vm”, plus being in sync only matters within your own cluster?