Guide about date

According to

```
9. [Mandatory] A table must include three columns as following: id, gmt_create and gmt_modified.

Note: id is the primary key, which is unsigned bigint and self-incrementing with step length of 1. The type of gmt_create and gmt_modified should be DATETIME.

```

Can I understand that in Alibaba it is recommended to define the date type as DATETIME? If not, shall we add guide about this? TIMESTAMP or DATETIME is the right choice?

We see that it is mandatory to define gmt_create, gmt_modified, can we understand that when dealing with cross time zone issues you recommend storing the date of gmt time in the database and then converting it to local time zone for display?

issue 中沒有指出檔案或測試。首先確定指南應推薦 DATETIME 還是 TIMESTAMP，以及應如何跨時區處理 UTC 值；當指南包含明確的建議和時區指引時，即表示完成。

由索引模型根據 Issue 內容生成。