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 ではファイルもテストも特定されていません。まず、ガイドで DATETIME と TIMESTAMP のどちらを推奨すべきか、またタイムゾーン間で UTC 値をどのように扱うべきかを明確にします。ガイドラインに曖昧さのない推奨事項とタイムゾーンに関する指針が含まれれば完了です。

索引モデルが issue の本文から書いたものです。