Local Time Ambiguity

Suppose a sunrise observation is made in Tokyo (latitude = 139.6500°), and sunrise is calculated as "21:47:35 GMT". Tokyo is GMT+9, so we'd say sun rises at "6:47:35". But shouldn't we use the "true" timezone of Tokyo, which would be "latitude/360*24" = 9.31 hours, yielding a sunrise at "7:6:11"? Which one is correct?