there will be challenges, of course. i just thought i'd surface this idea. a time valid range feels like an exception to caveats. technically caveats could be / have been (historically for expiry) used to handle time ranges by hoisting and delegating the complexity of managing the current time to the application. but i think it makes more sense that time ranges are completely handled by spicedb and caveats are more for passing domain specific context for edge cases where you need it.