ganuonglachanh How are you importing your data ? With PHP ?
Because if you're importing through SQL you can simply use timestamps and configure the timezone on source and destination servers.
If you're importing with PHP through a Flarum extension or a Laravel application you should be able to set the dates on the model. You can use Carbon\Carbon or DateTime objects on these properties.
To find the field names you can have a look at the migration files https://github.com/flarum/core/blob/v0.1.0-beta.6/migrations/2015_02_24_000000_create_discussions_table.php and https://github.com/flarum/core/blob/v0.1.0-beta.6/migrations/2015_02_24_000000_create_posts_table.php
The way you set the column depends on the method you use in PHP (Eloquent models in Flarum or Laravel, raw SQL commands, another ORM, ...)