1. en: add convert_tz.md
2. en: fix curdate.md, the fix is to modify the title
3. en: add curtime.md
4. en: fix str_to_date.md, the fix is to modify the title
5. zh-CN: fix convert_tz.md, the fix is to modify the description
6. zh-CN: fix curdate.md, the fix is to modify the title
Main CL:
1. Copy the code from BE to implement the `str_to_date()` function in FE.
2. `str_to_date("2020-08-08", "%Y-%m-%d %H:%i:%s")` will return `2020-08-08 00:00:00` instead of `2020-08-08`.
