Comment by pseidemann

The fix is storing the datetime without a timezone, so the hour stays stable, no matter what the timezone of Perth does, plus optionally and separately the location (Perth), which could be stored as the timezone id of the location. But you could also use coordinates. Anything which can be mapped to its timezone later works. Now the time will never change for the original location (sic), and you can always compute the time for any timezone in the world, because in the instance of computation, you take the current valid timezone of the location, and you apply that to the stable datetime. Of course a current computation of that might have a different value in the future. But that doesn't matter, since the time for the original location (sic) never changes, and the value for different timezones is supposed to change.