When the various torspec documents specify time, they refer to UTC. But the implementations used by at least Linux, *BSD and OS X are based on the Unix time epoch. This makes a difference to how leap seconds are handled: UTC includes leap seconds, but unix time excludes them. We should: * ensure that none of the security properties of tor depend on leap seconds either being present or absent, either individually or in aggregate: * every minute is not 60 seconds long (and equivalently for hour, day, week) * some epoch times can repeat or be missing * UTC and Unix time differ by approximately 30 seconds * check how the current Linux, BSD, Windows and OS X implementations handle leap seconds (in roughly that order of priority) * consider and document tor's handling of leap seconds See: * https://en.wikipedia.org/wiki/Leap_second * https://en.wikipedia.org/wiki/Unix_time