You need to remember the exact same bits of information in both cases, so I don't see the problem. In one case you remember it as "time difference" ("now is 12:00 +9 there"), in the other case you remember it as earlyness/lateness compared to you ("12:00 here is LIKE 12:00 +9 there").

So that would need the same exact effort.

The man benefit would be for the "let's meet at the same time online" etc coordination stuff -- where's its brain-dead easy when everybody has the same time.