Incorrect opened_at in table pull_request_history

[whystar]

I found this problem where I compared the opened_at of a pull-request in GHTorrent and the created_at of the same pull-request accessed via the GH official API.

Here is an example:

1. data in table pull_request_history

id	pull_request_id	created_at	action	actor_id
20920677	345020	2012-08-30 17:41:41	opened	654469

the PR is the #1266 in repository cocos2d/cocos2d-x

2. data fetched by GH official API
   https://api.github.com/repos/cocos2d/cocos2d-x/pulls/1266
   "created_at": "2012-08-30T19:41:41Z"

2012-08-30 17:41:41 in GHTorrent is two hours early than 2012-08-30T19:41:41Z in GH.

At first, I thought this inconsistence is caused by the difference of timezone. However, the created_at of a repository in GHTorrent equals to the value fetched by GH official API.