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 中没有指出文件或测试。首先确定指南应推荐 DATETIME 还是 TIMESTAMP，以及应如何跨时区处理 UTC 值；当指南包含明确的推荐和时区指导时，即表示完成。

由索引模型根据 Issue 内容生成。